Based on the radar image model of spaceborne parasitic SAR system, interferometric model of this new system is proposed; new characters and limits are analyzed in detail. The concepts of the range and azimuth effective baseline are given at the same time. The limits of effective baselines and performance of INSAR relative height measurement of Cartwheel and Pendulum configuration are analyzed. The influence of interrelation of the range and azimuth baseline and pre-filtering technology are pointed out.