Abstract: A high power microwave antenna was developed based on longitudinal shunt slots of rectangular waveguides. The antenna consists of 4 route rectangular waveguides and a Teflon window. To obtain high power capacity, the high power microwaves inside antenna transmit in vacuum system. To restrain the high power microwave breakdown in surface of window on the vacuum side, the window surface at vacuum side is notched periodically with triangle grooves. There are support planks between waveguides and window. The support planks not only support the window but restrain the mutual coupling of the slots of neighboring waveguides. The antenna was optimized by HFSS software. The antenna works at 1.575 GHz. With a gain of 22.7 dBi and aperture efficiency of 98.3%. The active reflection coefficients of 4 route rectangular waveguides are less than -25 dB, the reflection bandwidth is 5% and the fluctuation of gain of antenna is no more than 0.4 dB over the reflection bandwidth. The aperture efficiency of antenna is no less than 98% and the deviation of direction of main lobe is no more than 1.2 over the reflection bandwidth. The high power capacity of antenna is 1.92 GW.