Abstract: When a vortex beam is generated by liquid crystal spatial light modulator (LCSLM), the polarization direction of the incident light has a significant effect on generating the vortex. Based on the physical principle of the spatial light modulator, the influence of the polarization directions of the incident light on the output beam is calculated and analyzed, and the calculation is compared with the experimental results. The results show that, the modulation error of the LCSLM increases with the angle between the polarization direction of the incident light and the optical axis of the liquid crystal molecule, and the quality of vortex beam is worse. In particular, the quality of the vortex beam is significantly worse when the angle of the angle is greater than 4.725°. As the angle continues to increase, the modulation effect of LCSLM on the incident light is weakened, and the vortex beam can not be generated.