Based on the theory of semi-analytical thermal analysis, the thermal effects of the diode-end-pumped Nd:YAG microchip crystal with back surface cooling were investigated. A thermal model that matches actual working state of the laser crystal is established by analyzing the working characteristics of the Nd:YAG microchip crystal. Through using a new method to solve the heat conduction equation of isotropic material, a general expression of temperature field, thermal distortion field and additional optical path differences (OPD) within Nd:YAG microchip crystal was obtained respectively. Calculation expression of thermal focal length of the Nd:YAG microchip crystal was obtained by analyzing the additional OPD caused by heat. Research results show that a maximum temperature rise is 70.36 ℃, a maximum thermal distortion is 0.465 μm and a maximum additional OPD is 0.836 μm when the LD pump light is fourth rank of super-Gaussian distribution, the total power is 24.2 kW,and the duty cycle is 10%.