The thermal effect of Nd:YAG crystal rods induced by laser diode-pumped is analyzed theoretically, the more rational boundary conditions are established considering the distribution of laser beam in the crystal and the heat convection between the side surface of the crystal and the coolant. As a result, the more actual temperature field in the crystal is obtained, and the influences of crystal radius and side surface area on thermal effect are calculated simultaneously. The results show that the temperature in the center and on the side surface of the crystal increases when the convective heat-transfer about the side surface is considered, the focal length of crystal increase a little bit correspondingly. As decreasing the crystal radius, the differences in temperature between the center and the side surface of the crystal decrease, the focal length of crystal increases and the thermal effect of the crystal reduces. As increasing the side surface area, the temperature of the crystal decreases, while the focal length of crystal decreases and the thermal effect aggravates, for the rate change of temperature nearby the crystal center increases for the feature of end-pumped.