The Electron Cyclotron Resonance Maser with Quasi-Optical Cavity （ECRMQ） has been analysed systematically and series new conclusions have been achieved in this paper. Making use of the linearized Maxwell-Vlasov equation

the kinetic theory of ECRMQ has been worked out. Various schemes including those presented in this paper for building ECRMQ were investigated and the beam to weve interaction power

starting current and frequency shift for each of these schemes have been formulated; the high modes operation

and the field polarization were also discussed. A set of important problems concerning the ECRMQ have been analysed and discussed in detail. It is indicated that the ECRMQ has a lot of important characters and these new schemes presented in this paper are of significance.