The gain characteristics of the collective free electron laser operating in a superradiant amplifier have been studied through the variation of the interaction length and pumping magnetic field. We could obtain arbitrary interaction length by means of an axially movable "kicker" magnetic field that deflected the electron beam. The experimental results showed that the radiation gain of collective FEL was approximately 1.7th power of the pumping magnetic field with gain ranging from 0.4dB/cm to 1.38dB/cm.