According to the Weizsacker-Williams approximation, FEL Is simulated by collision between the relativistic electron and the pseudophoton, and the photon is scattered by the electron. According to this physical model, we studied the energy conversion between the electron and the photon, and obtained the relation between the energy of scattered photon and the energy of photon before collision:εp=4γ2εp(0). Increase of the energy of the scattered photon is at the price of the energy that the electron consumes. We calculated the scale of the energy conversion. In FEL, the maximum energy conversion efficiency is 50 %.