Based on analysis of the typical structure and system configuration parameters of grating lateral shearing interferometer, we systematically study the most significant errors of the interferometer system: geometric optical path difference and detector tilt error. We give the analytical expression of systematic errors before and after wavefront reconstruction in the form of Zernike polynomials. The relationship between the system errors and the measured numerical aperture (NA), the distance of diffracted light converging point d, the shear ratio s are quantitatively analyzed. The most important errors for differential wavefront of shearing interferometer are the coma and astigmatism of geometric optical path difference, the astigmatism and defocus of detector tilt. These error terms will cause the errors of spherical aberration and coma of geometric optical path difference, the coma and astigmatism of detector tilt through wavefront reconstruction. The error of reconstructed wavefront increases rapidly with the increasing of NA、d, but increases with the decreasing of s. Especially, the wavefront reconstruction has gained effect on system errors with small shear ratio (s≤0.05), and the system errors of wavefront reconstruction is much larger than the differential wavefront. The root-mean-square (RMS) of reconstructed wavefront error is much greater than 1nm under small shear ratio when d>2 μm, NA>0.1.