This paper focuses on the RF study of a C-band barrel open cavity (BOC) pulse compressor. The operating principle of BOC is presented and the technical specifications are determined. The main parts of BOC such as the cavity, the matching waveguide, the coupling slots and the tuning rings were numerically simulated by 3-D codes software HFSS and CST Microwave Studio (MWS). The "whispering gallery" mode TM6,1,1 with an unload Q of 100000 was chosen to oscillate in the cavity. An energy multiplication factor of 1.99 and a peak power gain of 6.34 was achieved theoretically.