The electron 1–D movement for the variable parameter Smith – Purcell FEL is discussed. When the laser operates in state of the low – gain, the analytical expression of the efficiency is derived and a reference formula as how to choose the change rate of the gratting parameter is also given. The numerical simulation results for a radiation output of 1m m ?wavelength are presented. The interaction length between the electrons and the field can be stretched and the efficiency will be enhanced when the change rate is chosed properly.