Abstract: The X-band high-gain 120 W transmitting module is developed by using three-stage power amplifier topology and gallium nitride (GaN) microwave power amplifier, and by designing the power amplifier radio-frequency (RF) circuit and control protection circuit. The RF circuit inside the transmitter module is deeply studied, including: high isolation GaN power amplifier bias circuit, GaN power amplifier cavity resonance simulation, high-directivity microstrip directional coupler and miniaturized microstrip step conversion circuit.The transmitting module test results show that, in the X-band frequency range of 80 MHz and under the conditions of input power of 10 dBm, 25 μs pulse width and 2% duty cycle TTL modulation signal input, the output power is greater than 120 W, the gain is greater than 40 dB, the top drop is less than 6%, the front and back edge of the pulse is less than 60 ns, and the spurious suppression is less than -62 dBc, the harmonic suppression is greater than 68 dB. The solid-state power amplifier module is fine in performance and can meet the requirements of system.