In the optical parametric oscillator (OPO) based on CdSe crystal, a large amount of waste heat is generated when the pump laser passes through the crystal, resulting in a significant thermal lens effect in CdSe crystal, which causes the spot radius of the pump laser smaller inside the crystal, and finally reduces the damage threshold of the crystal and the output power of the OPO. The thermal lens effect simulation of CdSe crystal pumped by high frequency pulsed laser is completed by COMSOL software in this paper. Through parameter optimization, it is found that the convection coefficient is inversely proportional to the maximum temperature of the crystal, directly proportional to the spot radius of the back face and focus of the crystal, and the focus position tends to be stable with the increase of convection coefficient. The single pulse energy and repetition frequency are directly proportional to the maximum temperature of the crystal and the spot radius of the focus, and inversely proportional to the spot radius and the focus position at the back end of the crystal. The collimated laser spot radius is inversely proportional to the maximum temperature of the crystal, and directly proportional to the spot radius, focus position and spot radius of the back end of the crystal. This study solves the problem that the laser spot radius at the back end of CdSe OPO crystal is difficult to measure directly, and provides a theoretical basis for optimizing the thermal lens effect of CdSe crystal.