The time and frequency synchronization error models are given according to the theory of spaceborne parasitic interferometric synthetic aperture radar（InSAR） and the work characteristic of ultra stable oscillator（USO）.The effect of time and frequency synchronization error on interferometric phase is analyzed based on the error models.The decorrelation caused by the time and frequency synchronization errors is presented for the first time.Based on the formation configuration and image signal model of the spaceborne parasitic InSAR system

the thermal noise decorrelation

ambiguity noise decorrelation

block adaptive quantization（BAQ） noise decorrelation

baseline decorrelation

Doppler decorrelation

mis-coregistration decorrelation and volume scatter decorrelation are analyzed.The relative height accuracy of the system is deduced when all the signal decorrelations are taken into account.The relative height accuracy of the seven sub-swaths of ASAR system on Envisat is simulated.The theory analysis and simulation results indicate the spaceborne parasitic InSAR system can achieve perfect performance if the orbit and radar parameters are correct designed.