Abstract: Microwave power combination is one of the most technologies and the research highlights in the future. The pulse width of high power microwave radar and ultra-wideband radar is between several ns and tens of ns. It is necessary to consider not only the pulse phase coherence but also the time delay jitter of the power unit when researching microwave short-pulse power combination technology. An estimation formula of power combination efficiency in time domain was studied by the method of probability in this paper，and the validity of the formula was proved by numerical simulation，which provides a reference for the design of microwave short-pulse power combination research.