Abstract: Based on the analytical solution of the dependence of phase matching angle on the crystal temperature, we illustrate the thermo-optic physical mechanism of temperature influence on the frequency conversion. The thermal sensitivity of the doubler and tripler crystals are numerically simulated respectively. Due to such sensitivity discrepancy among various crystals, the phase matching angle curve regarding the crystal temperature variation in our laser facility is obtained in experiment. The influence of PM angular detune on frequency conversion is theoretically analyzed under different input intensities, which is demonstrated by the conversion efficiency decline induced by temperature variation. According to the thermal sensitivity parameter obtained in experiments, we compensate the conversion efficiency with the crystal angle following, resulting in stable output of third harmonic energy.