Abstract: The influence of uncaptured electron on energy conversion efficiency in multiphoton nonlinear Compton scattering has been studied by using a particle theory and the inelastic collision model about electron and photon. It is found that the expansion width of the circular frequency optical spectrum of the scattered photons in multiphoton nonlinear Compton effect increases with the incident electron velocity increase and the increase of the photon number by the electron absorpted at the same time，and it reduces when the inelastic collision between electron and photon increases. The expression of the direct ratio lowering of the energy conversion efficiency by electron incident speed in multiphoton nonlinear Compton scattering and a method of lowering the loss with low energy electron i