A new calculation method is proposed for studying the characteristics of liquid crystal spatial light modulator (LC-SLM) when the incident readout light is oblique. The theoretical model is established, and the simulative results are given. According to the relationship of driving voltage to intensity of readout light at normal incidence, rotation angles of LC-SLM molecule with different modulation voltages are deduced and modulation depth is calculated. Phase modulation depth decreases with the incident angle increasing, while phase modulation depth increases with the obliquity of LC-SLM molecule decreasing, i.e., driving voltage of the LC-SLM increases with the intensity of write-in light increasing. With the incident angle increasing, initial phase of readout light decreases, but the maximal initial phase is 0.21π at the incident angle of 10°. The simulative results accord with the experimental data.