A set of steady temperature measurement device was developed. A high accuracy of ±1℃ could be obtained by the system as long as the error was calibrated. The piston temperature was measured at the engine speeds of 1000r/min, 1500r/min and 2100r/min on the external characteristics of the engine of EQD180N—20, respectively. The piston temperature variation test was performed from 1000r/min to 2100r/min simultaneously. The results show that the proposed system is stable and reliable. On the external characteristics, the temperature of each feature point increased firstly and then declined slightly as the engine speed increased, and a maximum value was observed at 1500r/min; on the stable working condition, the highest temperature appeared in the piston top surface with a value of 264℃, The first piston ring slot temperature is about 215℃.