Using a disturbance develop method, we have obtained the formula for the small signal gain (S. S. G.) of the variable parameter Smith-Purcell effect FEL and show that the gain spectrum function factor takes the same form as the variable parameter Wiggler.On the basis of this formuls, the S.S.G. for linerly varying of grating space frequency is discussed. The resultS show that when the speed of the injected electron is within a range,lower than the initial resonance speed,a higher S.S.G. can be attained, and that in order to enhance S. S. G. for a given saturation efficiency, the changing rate of grating space frequency should be chosen properly.