The synthetic aperture radar（SAR） imaging system is portrayed as a multiple access information system and an information theory approach is applied to the SAR imaging system.The information content about a point target that can be extracted from its radar images is evaluated by the average mutual information and the average mutual intormation is also used to define a measure of radiometric resolution for radar images.A translative probability density function of the SAR imaging system is derived by analysing the system and a closed form of the information content is found.Some conclusions are obtained.The results presented provide a theoretical interpretion and an explanation for some well-known results in the field of SAR.