Based on the framework of spaceborne parasitic SAR system and considering spacebornebi-staticSAR configuration formed by the existing satellite and eachmicro-satellite, a geometrical model is established, and formulations of spatial resolution in range and in azimuth are deduced. Finally varieties of the spatial resolution in range and in azimuth along with the angle of incidence, height of the relative orbit and the range between the existing satellite andmicro-satellite are simulated and analyzed. The results indicate: (1) the spatial resolution in range and in azimuth are very similar to the existing satellite's； (2) the range between the existing satellite andmicro-satellite is longer, the spatial resolution is bigger； (3) the height of the relative orbit is greater, the differences of spatial resolutions of eachmicro-satellite are bigger.