Active phased array radar (APAR) services play a key role in national strategic security equipment and directly support essential tasks such as the national strategic missile defense, over-the-horizon detection, anti-stealth detection, and remote guided attack. APAR technology was used during war in the 1960s and became very popular worldwide because of the emergence of urgent military needs. This technology has essentially affected the world military. Compared with traditional single pulse and pulse Doppler radar technology, APAR has helped to advance radar technologies and had a profound and wide influence on the development of radar. Each antenna element in an APAR is connected with corresponding transmission/reception (T/R) modules. By controlling the phase shifter to change the phase distribution on the antenna aperture, the antenna can be electronically scanned without any mechanical rotations while still covering the whole airspace. Therefore, compared with the disadvantages of traditional mechanical scanning radar, such as a large scanning inertia, limited data rates, a small number of information channels, and difficulty meeting the requirements of adaptation and multifunctionality, APAR has unparalleled advantages such as flexible and non-inertial scanning completed within microseconds, multifunctionality, high reliability, large data rates, low radar cross-sections of radar reflection, strong adaptability and less affected by electromagnetic interference. With the significant demand of modern national defense, radar equipment is continuously being developed with over-the-horizon techniques, accurate detections, strong stealth capabilities, etc. APAR research is advancing toward high-frequency bands, high gains, high pointing accuracies and low sidelobe levels. The high electromagnetic performance of antennas is achieved through strict design parameter requirements such as rigidity, quality of being lightweight and high efficiency heat dissipation of structures. Moreover, various parameters of antennas show high-dimensional and multi-field coupling relationships. It is more sensitive to interference from harsh battlefield environments, which deteriorates the electric performance of antennas and reduces the detection power, guidance accuracy and battlefield survival ability of radars. APAR, known as the eye of the three armies, is a piece of typical equipment involving interdisciplinary disciplines. Its structure, thermal and electromagnetic interactions and mutual restriction coupling relationship are defined as the structural-electromagnetic-thermal (SET) coupling problem of APARs. The main coupling problems include the following. (1) Feed errors affect the antenna electromagnetic performance, including the amplitude and phase errors of the feed network of APARs; failure of the radiation element, temperature drifts of the thermal sensitive electronic components (such as phase shifter in T/R module) and mutual coupling of the antenna elements will cause the amplitude and phase errors of the feed current, which will lead to the deterioration of antenna electromagnetic performance. (2) The structure errors affect the antenna electromagnetic performance, and there are random errors in the manufacturing and assembly of APARs. Vibrations, shocks and thermal power consumptions during service cause deformations of the antenna array, which eventually results in position offsets of the radiation element and changes of the electromagnetic amplitude and phase distribution on the antenna array. Moreover, this results in the change of the transmitted beam and finally, seriously affects the antenna electrical performance. (3) Thermal issues affect the antenna electromagnetic performance, and thousands of T/R modules, which consume large amounts of thermal power, are installed on APARs. On the one hand, it will cause thermal deformation of the antenna array structure; on the other hand, it will also cause performance degradation of the device and finally lead to the deterioration of antenna electromagnetic performance. (4) Changes in the coupling ofstructural, thermal performance and electromagnetic performance will cause deterioration. In APARs, the electromagnetic amplitude and phase of the antenna array will distribute correspondingly under different duty cycle operating modes, which results in a change in the thermal power consumption and temperature distribution. Thus, the thermal deformation of the antenna array structure is affected. Therefore, the structural-electromagnetic-thermal coupling problem of APARs has become a bottleneck issue that hinders their steady development and further enhancements of their performance. In this paper, the development of APARs on different ground-based, shipborne, airborne, missile-borne and spaceborne platforms has been sorted, and the structural characteristics of APARs on each weapon platform have been analyzed. The influence of service loads on APARs in different battlefield environments of land, ocean, air and space has been summarized. Then, the mechanism analysis and modeling method of SET coupling of APARs affected by antenna structure errors, high-temperature ablations of the radome, feed errors of T/R modules and failures of antenna elements are discussed. Moreover, the application of SET coupling technologies in the fields of APAR manufacturing accuracy, high-efficiency heat dissipation, lightweight integrated optimization, sparse array design, etc., as well as the key guarantee technologies in service environments such as APAR condition monitoring, displacement field reconstruction, electrical performance compensation, are summarized. Finally, future research on SET coupling technologies and their application prospects in different research fields are discussed.