Abstract: In this paper we used the one-dimensionala non-steady state FEL equations developed by Prof,Yu Min to make some numericall simulations,We investigated the FEL pulse propagation,gain development and superradiance in the slippage section in the linear regime,It is shown by the numerical results that the effects of slippage on the gain is related to the longitudinal distributions of the electron beam and the ratio of the slippage length and the electron pulse length,and the gain in the slippage section, relative to that in the steady state section,is determined by the beam current for the same slippage parameter.