Abstract: Microwave power spatial synthesis from multi-source is an effective and widely used method to realize high power microwave radiation. In many cases, output power of high power microwave sources is with random phase shift because of their unstable property. This paper employs numerical calculations, deals with the phase shift of microwave sources which are even distribution and calculates the synthesis efficiency using the average gain of radiation antenna arrays. By numerical calculations, the impact of the coupling coefficient between antenna elements, the type and polarization of antenna arrays, and the antenna gain on the synthesis efficiency are analyzed. The analysis shows that the coupling between antenna elements may seriously reduce the synthesis efficiency; the effect of antenna t