The influence of the adiabatic-magnetic-field distribution at the entrance of wiggler on the output power, gain, and efficiency in a free-electron laser is investigated in terms of nonlinear simulation. Results show that the distribution f(kwz) = (kwz/2πNw)3 is better and closer to the measured data than both f=sin2(kwz/4Nw) and f=(kwz/2πNw)2.