Abstract: In the high-power millimeter wave systems of hundreds of kW or MW, the output millimeter wave power is generally measured by calorimetry. According to the needs of power measurement of hundreds of kW long pulse high-power millimeter wave systems, a power measurement system based on calorimetry is designed in this paper. In the proposed design, the incident millimeter wave energy is converted into heat by absorbing load. By monitoring the temperature and flow at the inlet and outlet, the power measurement of high-power millimeter wave is realized, and repeated experiments are carried out. To trace the quantity value and improve the accuracy of the measurement system, the energy standard device is designed, and the system energy measurement error is deduced to calibrate the measurement systems.