Spaceborne InSAR (Interferometri Synthetic Aperture Radar), whose baselines may undergo complicated variations while the satellite formation flies in the space, is widely used in earth observation. Based on the principle of satellite formation, the even phase configuration for the spaceborne InSAR was defined. Then baselines and their stability were supplied from the principle of InSAR measurement, and the stability of spaceborne InSAR in different even phase configurations was addressed.