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