The FEL oscillators with highp ower, highefficiency require stable electron beam as a pump. That means e nergy, current and period of ebeam should stabilize pass by pass during a mac ropulse. Actually,it is impossible to keep an absolute stabilization of electron beam during a micropulse. The jitter of ebeam will affect gain of laser seri ously. The effects of energy jitter of ebeam on the gain are studied using the FEL 1D code (W1D) developed by us based on the parameters of the CAEP FIR-FEL. The results show that the energy jitter should be less t han 0.5% for CAEP FIR-FEL.