根据Ｗｅｉｚｓａｃｋｅｒ－Ｗｉｌｌｉａｍｓ近似，把自由电子激光模拟成相对论性电子与赝光子的碰撞，而光子被散射．按照这个物理模型，研究了电子与光子的能量交换，得到了散射光子能量与碰撞前光子能量之间的关系：散射光子能量的放大是以电子消耗能量为代价的分析。计算了能量交换的规模，并得出自由电子激光中能量转换效率最大为５０％ According to the Weizsacker-Williams approximation, a free-electron laser is modeled as a collision of relativistic electrons with a photon, which is scattered. According to this physical model, the energy exchange between electrons and photons is studied, and the relation between the energy of the scattered photons and the energy of the photons before the collision is obtained. The energy of the scattered photons is amplified at the expense of the energy consumed by the electrons. The scale of energy exchange was calculated, and the maximum energy conversion efficiency of free electron laser was 50%