Based on the 0.15 μm GaN High Electron Mobility Transistor(HEMT) technology, a high linearity power amplifier Monolithic Microwave Integrated Circuit(MMIC) used for 5G communication is designed and fabricated. By optimizing the structural material, the amplifier has a relatively smooth transconductance characteristic over a wide gate voltage dynamic range. By optimizing the setting of the static DC operating point, the output power and linearity requirements are balanced. The three-stage amplification structure with a gate width ratio of 1:2:3.2 ensures the gain and power index of the circuit. The on-wafer test results of the chip show that the small-signal gain of the MMIC is greater than 22 dB; the saturated output power is 40-40.5 dBm with above 18 dB power gain and a power added efficiency is 30%-33% over the band 24-28 GHz under testing condition of 20 V drain voltage, 100 μs pulse ,10% duty circle; when the output power is back to 34 dBm, its Power Additional Efficiency(PAE) is 18%, and the third-order Intermodulation Distortion(IMD3) is less than -30 dBc under 26 GHz with the 100 MHz frequency offset. The chip size of the MMIC amplifier is less than 3.4 mm×3.2 mm.