The high power microwave (HPM) synthesis method is presented in this paper for gigawatt level. The gigawatt level HPM could be synthesized from two separate input wave-guides according to the coupled-wave and orthogonal polarization theory. The synthesizer is used by two back to back circular wave-guides. The main channel is the circular wave-guide connected to the output port, which transmits horizontal polarization TE011 mode. The operating bandwidth is only limited by the barrier wave-length λc of circular wave-guide. The sub-channel transmits vertical polarization TE011 mode and the operating bandwidth is up to several hundred MHz. The energy of sub-channel could be coupled into main channel through continuous long-slit coupling structure. The synthesizer can be analyzed using numerical simulation method, which focuses on the power capability. The simulation results indicate that the transmission efficiency of the main channel is above 99%, the coupling efficiency of the sub-channel is above 96%, which also validates the reasonability of synthesizer design. At the same time, the prototype of synthesizer is designed and the HPM experiment system is established. The transmitting and coupling efficiency are both greater than 95% in cold test condition and they are also greater than 90% in gigawatt class test condition, the power capability of the synthesizer reaches about 1.2GW. The test results validate the feasibility of synthesizer for gigawatt class HPM.