To meet the high power capacity and compact application requirements of microwave output window in high power microwave system, a C-band miniaturized high power microwave output window is designed based on the design theory of traditional box window by optimizing the form structure and adding transition segments. By increasing the surface area of the window and changing the connection mode of the rectangular waveguide-circular waveguide transition segment, the power capacity can be increased and the longitudinal size of the microwave output window can be reduced. The “I” form structure can effectively suppress the influence of multipactor near the triple point(vacuum-media-mental) on the performance of the output window. On the basis of electromagnetic simulation, the multipactor near the triple point of the microwave output window is studied by using the Particle-in-Cell method. From the microscopic point of view, it is further confirmed that the “I” form structure can make the position of the triple point move, reduce the probability of multipactor generated by the electrons emitted by the triple point on the surface of the window, and reduce the breakdown risk of the microwave output window. The design results show that the main mode reflection coefficient of the microwave output window at the center frequency is lower than 0.01, the transmission efficiency is higher than 99.9%, and the power capacity can reach 47.9 MW.