Based on the theory of semianalytical thermal analysis, temperature and thermal distortion held, additional optical path difference (OPD) and thermal lens within a diode-end-pumped Nd:YAG microchip crystal with back surface cooling were investigated. Expressions of temperature and thermal distortion field, additional OPD and thermal lens were obtained through the analysis on working characteristics of the Nd:YAG microchip crystal. Expression of the thermal focal length within the Nd:YAG microchip crystal was obtained through analysis of the additional OPD caused by heat. Research results show that the maximum temperature rise on the pump face of Nd:YAG crystal is 24.3degC; the maximum thermal distortion is 112 nm; the additional OPD is 201 nm and the thermal focal length is 0.984 m when the output power of LD with Gaussian distribution is 10 W. This method can be applied to other thermal analysis of microchip laser crystal and offers a theoretical basis for the optimized design of diode-end-pumped microchip laser with back surface cooling.