Abstract: In this paper, an actual and effective realization of the principle of the FEL operation in a longitudinal uniform magnetic field is discussed. It recommends a synchronous spiral quasi-static electro-magnetic field and a synchronous spiral resonant cavity electro-magnetic field to be used as two kinds of energy compensation of FEL in a longitudinal uniform magnetic field, and a synchronous spiral static-electric field as a starting gyration and tapering gyration for electron beam. A brief description is given to their function, characteristics and feasibility.