Spatially modulated imaging polarimeter (SMIP) system is able to modulate four Stokes parameters S0-S3 in a single interferogram by using Savart plate. Through sliding reconstruction method, the entire polarization information can be demodulated from the interference fringes. In this system, the alignment errors of the half wave plate (HWP) and the analyzer has a non-ignorable impact on the measurement accuracy of the Stokes parameters. A theoretical model including these two alignment errors were presented in this paper. Based on this model and parameters of the prototype SMIP, the Stokes parameters measurement accuracy of unpolarized light, 0°/90°linear polarized light, 45°/135°linear polarized light and left/right circularly polarized light was analyzed. According to these four basic polarized lights, a method for solving Stokes parameters measurement error of incident light with any state of polarization (SOP) and degree of polarization (DOP) was given. In order to alleviate the influence of HWP and analyzer alignment errors, an optimization using condition number of measurement matrix as an objective function was given. It is shown that when the thickness of Savart plate is 23 mm, which is corresponding to the minimum condition 2.06, measurement errors induced by HWP and analyzer alignment errors can be effectively diminished.