Abstract: The stability of the free electron laser output is mainly determined by the stability of the electron beam, an d the stability of the electron beam based on RF linac is mainly determined by t he stability of the microwave source. To realize the saturation output of FEL an d to get better FEL spectrum purity, the amplitude and phase stability of RF power must be very high according to FEL theory. The FEL stability and its requirements on microwave stability are discussed, and the required parameters of microwave stability for CAEP FIR-FEL and BFEL are obtained.