The active decoy jamming to SAR based on the Time-Delay Doppler-Shift (TDDS) method is effective in certain regions. Proper utilization of jamming to ensure good decoy quality requires a study of the effective region. After the mathematical analysis of the difference between the jamming signal and a real point-target echo, the paper points out that residual Range Cell Migration (RCM), matched filter error, and loss of Doppler bandwidth are three main factors that lead to a deterioration of the focus of a jamming signal. The formulation of the effective regions is obtained and verified by simulation results. The study indicates that the TDDS method can effectively protect limited regions around the jammer.