Traditional interferometric synthetic aperture radar (InSAR) is based on broadside looking geometry and parallel tracks. With the increase of the orbit height in spaceborne SAR and the development of SAR constellations, InSAR data of a region can be acquired in complex geometry, especially squint beam steering and unparallel tracks. For the sake of optimal InSAR system design and data processing, it is necessary to model the geometric decorrelation in complex geometry. This letter derives an accurate analytical model of geometric decorrelation of SAR interferometric pairs for general SAR observation geometry. Nonidentity of impulse responses and nonorthogonal sidelobes are the main features hindering the model derivation in the complex geometry case. An impulse response-fitting method is proposed, where nonorthogonal bases are adopted to suit the features and, thus, accurately analyze the geometric decorrelation. Simulation results verify the analytical model. It is found that unparallel tracks will introduce an extra geometric decorrelation factor. Compared to cases of parallel tracks, unparallel tracks always worsen the geometric decorrelation and cannot be neglected.