By modeling with COMSOL software, we carry out the thermal analysis of optical parametric oscillator (OPO). We find that under high pump power, temperature near the central axis of crystal increases due to the absorption of parametric light. Because the refractive index of MgO∶PPLN crystal is very sensitive to temperature, a small temperature rise will cause the non-negligible wavefront distortion of parametric light, which leads to the decrease of beam quality. The temperature rise of crystal is caused by the absorption of parametric light, and such temperature rise is unavoidable. The deterioration of beam quality in high power OPO/OPA (optical parametric amplifier) is inevitable.