Spatial light modulator is the key component for dynamic computer generated hologram (CGH) and phase type liquid crystal on silicon (LCoS) device is among the most commercialized. Due to the consideration on efficiency or the requirement from system architecture in the holographic displays, the reconstruction illumination could obliquely incident on the LCoS, which will cause distortion of signal window, hence the image within the signal window. Therefore, the target image needs pre-distortion before the calculation and optimization for the corresponding phase pattern on LCoS in order to obtain the originally desired pattern. This paper illustrates the mechanism of image distortion and provides quantitative analysis for arbitrary azimuthal and radial angle of reconstruction illumination on LCoS device. The required correction is also derived with the corresponding experiment showing the effectiveness of the proposed scheme.