Abstract: Microwave power sensor is the primary instrument for measuring microwave power. To ensure the accuracy and consistency of the values, the power sensor needs to be calibrated periodically to trace to the national standards. In order to realize the automatic calibration of the power sensor, an automatic calibration system based on the transmission standard method was designed, the hardware composition and software design method of the system were described, and the automatic calibration and the automatic calculation of the calibration factor and its expanded uncertainty were realized.