Abstract: A code for simulation of 3D FEL oscillator has been established on 386 PC.Using this code,several calculations were done as follows:1.In single mode cases,(a)the influ-ences of non-ideal injiction,emittance and energy spread on FEL saturated power in a certainregion arenot distinctive, moreover,they become weakened when output coupling is small.Speciallyin our calculation with output coupling 2%, the saturated power for slight degree of non-ideal in-jection exeeed that of ideal injection.(b)the influenees of non-ideal injection on optical beamquality are not severe,they also become weakened when output coupling is small.2.Inmultimodes(16)cases,the calculations show that side bands can not grow up when outputcoupling is large,and non-ideal injection, emittance can slow down the growth of side bands.