Abstract The cyclotron autoresonance maser (CARM) has been developed which is a high power device operating at short wavelength. In this paper, the basic principle of CARM is described, and the design method and numerical calculation results for Bragg cavity are shown. Based on the distributed reflection performance of the corrugated waveguide, an important parameters S is found with application of coupled-wave theory. S parameter is used simply to design a Bragg cavity and to select one of the operation modes. Finally some discussions are made.