The invention discloses an intensity absolute calibration method of a microwave heterodyne system in a fusion device. A standard intensity source is adopted to provide radiation intensity to the microwave heterodyne system to be calibrated. The intensity absolute calibration method is characterized in that a chopper is arranged and adopted between the standard intensity source and the microwave heterodyne system to be calibrated. The chopper modulates a signal received by the microwave heterodyne system to be calibrated and provides a periodic signal and a reference signal. A phase-locked amplifier and an acquisition system are also adopted. The reference signal is directly inputted to the acquisition system. The periodic signal is inputted to the phase-locked amplifier. The microwave heterodyne system to be calibrated receives the signal modulated by the chopper and outputs the signal in two paths, wherein one path is directly inputted to the acquisition system and the other path is inputted to the phase-locked amplifier. The intensity absolute calibration method is concise and effective and easy to carry out so that intensity absolute calibration of the microwave heterodyne system is solved.