Based on a 0.25 μm GaN High-Electron-Mobility Transistor(HEMT) process platform on SiC substrate, a compact, high-power, high-efficiency Microwave Monolithic Integrated Circuit(MMIC) Power Amplifier(PA) is designed using electro-thermal co-design and microwave device/circuit simulation techniques. Based on output power requirements and load-pull characteristics,the dimensions and optimum impedances of the transistors at each stage are determined. According to the circuit gain requirements, a three-stage cascaded configuration is adopted, with total gate width ratios of 1:3.6:16 for the pre-driver, driver, and final stages, respectively. Through electro-thermal co-design, the die layout is optimized to reduce thermal coupling effects and enhance the heat dissipation capability of the circuit. By employing electromagnetic parasitic parameter extraction technology, a compact circuit layout is achieved with a chip size of 2.7 mm×5.6 mm. Test results demonstrate that the power amplifier achieves a saturated output power greater than 46.5 dBm, a Power-Added Efficiency (PAE) greater than 42%, and a saturated power gain greater than 26.5 dB over the 14~18 GHz frequency band. Owing to its excellent electrical performance, it is suitable for wide applications in communications and electronic countermeasures.