According to Diesel Engine ECU drive circuit, the drive current of each components measured with the Active Current Clamp was analyzed. According to the drive current of Solenoid pump valve, the heat dissipation of each component was calculated, and then the thermal simulation model was built and the ECU temperature calculated. In addition, an experiment was done to verify the accuracy of the thermal simulation model at 25°C environment. The experiment shows that simulation balance results is almost the same as experimental balance results, there are no more than 1°C difference between simulation results and experimental results, verifies the accuracy of the thermal simulation model.