By the nonlinear theory model of gyrotron traveling wave amplifier in which the influence of velocity spread is contained, the effect of velocity spread on the interaction between electron beam and wave for a W-band gyrotron traveling wave amplifier was studied. The results indicate that the influence of velocity spread in amplifier can be weakened by regulating the interaction length, decreasing the velocity ratio, adjusting the operating magnetic field. It provides the theoretic basis for the optimized design of gyrotron traveling wave amplifier.