An X-band 300 W inter-matching GaN High Electron Mobility Transistor(HEMT) with high efficiency based on domestic GaN epitaxial material is studied. The design of matched power devices in 300 W X-band is carried out by using the large signal model of large gate width chip, package shell model, bonding wire model, capacitance model and so on. Four-cell matched synthesis circuit is adopted, L-C network is utilized to improve the impedance of the device, and then impedance transformation and power synthesis are carried out throughλ/4 impedance transformation network to achieve impedance matching of 50 Ω. The power splitter and matching capacitance are realized by using high Q ceramic substrates. The internal matching device is developed with output power over 300 W, the gain above 9 dB and Power Adder Efficiency(PAE) greater than 38.9% in frequency from 9.5 to 10.5 GHz. At the same time, the variation of output power and PAE with operating voltage, pulse width and duty cycle is also studied.