Abstract: In order to realize accurate power measurement and effective protection of the transmitter, the principle of directional coupler, and the method of parameter calibration and power measurement applied to the RF transmitter system are described. According to the measurements, the method has the advantages of simple installation, high precision, and dynamic calibration. After parameter calibration, the output power and standing wave ratio are measured online by installing a coaxial directional coupler between the RF power source and the resonant cavity. The measurements are then sent to the system of control, protection and monitoring to complete high-power measurement and protection of devices.