Abstract: In the common method of high power amplifier design, the device is set to the state of absolute stability. However, this method will decrease the gain and maximal output power of the device. In this paper, an improved design method is proposed and validated which could ensure both of the stability and gain of the circuit. The improved method is used in the design of a GaN HEMT high power amplifier firstly and the measured results show that the method is correct and practical.