Abstract: The traditional microwave source is mainly realized by electric vacuum devices such as TWT, magnetron, and BWT, etc. The high voltage, large power consumption and large volume of these electric vacuum devices result in problems of safety, efficiency, and portability. Microwave solid-state source is gradually replacing the traditional microwave source because of its high efficiency, good harmonic suppression performance, high stability and so on. In this paper, a compact high-power microwave solid-state source is designed, which uses the phase-locked loop as the signal source, the design amplifies the microwave signal step by step by the three-stage power amplifier, and achieves the output microwave signal with the power level of 300 W at 915 MHz. The measured results show that the working efficiency is larger than 70%, the second and the third harmonic suppressions are better than -40 dBc and -50 dBc respectively. The design has a good application prospect in industrial microwave applications such as microwave heating and thawing.