The GMTI (Ground Moving Target Indication) performance for circular scanning SAR (Synthetic Aperture Radar) equipped on slow platform was analyzed in this paper. The characteristics of SAR equipped on slow platform and the GMTI application were descript in the beginning, and the circular scanning mode was advanced to resolve the low azimuth imaging efficiency of the slow platform borne SAR. Then the range models of the stationary and moving targets based on circular scanning mode were built, and the Doppler frequency expressions were derived from the range models. The detectable velocities distribution areas were derived by separating the Doppler frequency of two kinds of targets. The quantitative computer simulations were given to demonstrate the detectable velocities area along with azimuth angle and pulse repetition frequency in the end.