In order to solve the problem of narrow working band and low output power and poweradded efficiency in current wireless communication system, this paper proposes a new type of the broadband high power amplifier in L Band designed by GaN device made by CREE. A method based on sourcepull/loadpull simulation has been used to find optimum source and load impedances .Then using an approach of mixing of microstrip line and capacitor,wideband matching networks is designed. Largesignal measurement results show that when the input power is 34 dBm among the band of 300 MHz in the Lband (relative operating bandwidth is 27.7%), the output power is higher than 50.4 dBm (108 W) , the poweradded efficiency (PAE) is over 48% and the plainness is ±0.1 dB at the whole working band. The data indicates that the power amplifier based on GaN has the advantages of wide band, high efficiency and output power.