In this paper, an energy model of FEL was set up to calculate the energy tranfer rate of FEL under the conditions of small signal and strong signal respectively. This model was simple in calculating, clear in physical meaning and adequate for FEL with different magnetic structures, such as FEL uwith wigglers, FEL with axial magnetic field FEL with helical axial magnetic field combination. It could be used to deal with various problems such as harmonc radiation, short wavelength radiation, parameter variations and so on. The model is suitable for three dimensional problems.