According to different ways of heat transfer, the needle valve body was divided into three areas. The way of heat transfer and the determination method of boundary conditions were analyzed. The internal flow model of nozzle hole was established based on the fluid volume method. The internal velocity distribution of nozzle hole was calculated and analyzed and the heat transfer boundary condition for the inner surface of needle valve body was determined. Then three-dimensional combustion model of diesel engine was established, the heat flux for the outside surface of the top end of needle valve body was calculated and analyzed and its heat transfer boundary condition was determined. The temperature distribution of needle valve body at maximum torque point and rated condition point was finally calculated through the fluid-solid coupling method. The results show that the temperature of needle valve body gradually increases from the top to the bottom and the temperature at the top is the highest. The temperature of nozzle hole gradually increases from inlet to outlet and the nozzle outlet temperature is the highest. The temperature of lower side is higher than that of top side for nozzle hole on the outside surface of needle valve body, which is equal to or higher than the temperature of carbon deposit formation like the nozzle outlet temperature. The temperature of needle valve body at the rated condition point is higher than that at the maximum torque point. It is known that carbon deposits form, increase and aggregate continuously at the outlet of nozzle hole under the mutual action of the needle valve body temperature and the high-temperature gas inside cylinder according to the actual distribution characteristics of carbon deposit in the needle valve body.