Abstract: Overmoded RBWO is utilized more and more often for its high power capacity. However, the both sides of SWS of overmoded RBWO consist multi TM0n modes, in order to achieve the design of reflector, it’s essential to make clear of the mode composition of TM0n. NUDT had done research of the mode composition in overmoded O-type Cerenkov HPM Oscillators in detail, but in the area electron beam exists, the influence of electron beam must be taken account of. Hot-cavity dispersion equation is figured out in this article first, and then analyzes the hot-cavity mode composition of an X-band overmoded RBWO tentatively.