In order to achieve broadband millimeter-wave amplifier with high gain and efficiency, GaAs pHEMT technology is employed for obtaining high gain; four-stage circuit topology is utilized for expanding bandwidth; current reuse technology is adopted for reducing DC power consumption; and reactance matching technology is implemented for optimizing output power and efficiency. As result, 31-38 GHz GaAs medium power amplifier Monolithic Microwave Integrated Circuit(MMIC) is successfully realized with broadband and high efficiency. In the frequency form 31 GHz to 38 GHz, the test results show excellent performances with 26-29 dB gain, 21.5 dBm output power, more than 37% Power Added Efficiency(PAE) and only 100 mA dynamic current. Especially, the efficiency of power amplifier has reached 45% at frequency from 32 GHz to 35 GHz.