Aiming at the problem of on-line temperature measurement of resistance element under extreme conditions, a live temperature measurement device based on thermocouple is developed. The structure of the device was optimized to solve the problems of heat transfer and insulation between the temperature probe and the resistance element, as well as the sealing and strength of the device itself. The temperature measurement values of the device at Mach numbers 5 and 6 were obtained through experiments, and the measured values at steady state were compared with the calculated values and the incoming flow temperature values. The results show that the measured values of the device are true and credible, which ensures that the resistance element is in the safe operating temperature range, and provides a basis for improving the state monitoring of the heater resistance element.