The dynamics model of electronic (active) decoys in exo-atmospheric flight is derived and the corresponding mathematical characteristic is analyzed in detail.It is found that the orbit of decoys is determined both by the radar position and the delay parameter of jammer.The decoy at every time instant still follows the law of elliptical orbit

with the semi-major and semi-minor axes in accordance with the physical targets

but the instantaneous elliptical center precesses around the Earth center.The delay parameter of the jammer determines the maximum radius of the precession while the radar position determines the varying angle of precession.This analysis can provide both countermeasure sides with theoretical basis when selecting a suitable countermeasure method.