In order to obtain high accuracy DEM products, distributed InSAR satellite system has a high requirement for the accuracy of inter-satellite baseline measurement. Baseline determination mainly consists of two procedures: GNSS baseline estimation and baseline calibration. In this paper, characters, mechanisms and accuracies of errors induced by baseline determination are analyzed. Simulation results of GNSS baseline estimation show that the random part of errors has a less impact which is only about 1 mm, while the systematic part plays a more important role in the impact of all errors. But it should be noticed that the systematic part, mainly comes from the error of antenna reference point, can be calibrated and compensated as a constant, while the random one cannot. Simulation results of baseline calibration show that: compared with interferometric phase noise and height references error, slant range distance error is the main error. The impact of slant range distance error on baseline calibration is nearly 2 mm, which can't be ignored.