Abstract: It is well known that the efficiency of constant-variable wiggler free electron laser amplifier is only several percent of the incident power. In order to obtain high efficiency of laser output, the wiggler must be tapered. As we know, under the longitudinal periodic force, the electrons trapped in ponderomotive potential become "bunched" and form "electron ensemble". When the output power of laser gets saturated, the average energy of the trapped electron ensemble reaches the minimum, (relative to the resonant energy). After that , the energy of electron ensemble rises, meanwhile the power of laser goes down. The trapped electron ensemble makes synchrotron motion in the ponderomotive potential. The main design idea of tapered wiggler is to keep the ensemble in the energy -lose state as long as possible by adjusting the resonant energy when the average energy of the ensemble reaches its minimum. In this paper, we have analysed the design methods of tapered wiggler with the previously proposed, "single particle resonance" and "exponential decreasing method of magnetic field", and then propose a new design method of "consistent design with multi -electrons and optimal selection" . The 3 -D simulation results show that our method is much better than the others.