Abstract: Heat generated by the absorbed pump light always results in a non-uniform distribution of temperature in the laser crystal, which leads to a non-uniform distribution of refractive index in the crystal. Therefore, a phase modulation can be produced when the oscillating beam passes through the crystal, thus the oscillating light field can be redistributed and deviated from the ideal mode distribution. Because of the filter function of the resonator, the resonator filters out the beam profile that does not match the ideal resonator mode and causes loss. By calculating the thermal loss of the fundamental mode and the conversion ratio of the low-order mode to the high-order mode, the temperature field distribution in the crystal was simulated with MATLAB, the energy interchange between the modes in the resonator was studied, which could provide a theoretical basis for suppressing the high-order mode and reducing mode loss in solid state laser.