The research subject is the amplification characteristics of the plasma-beam superheterodyne free-electron laser of the dopplertron type. The purpose is to theoretically investigate the influence of longitudinal magnetic field on the wave amplification in such a device in order to find the optimal operation mode. As initial we use the quasihydrodynamic equation, the continuity equation and Maxwell’s equations. The motion problem is solved by means of the modernized method of averaged characteristics. The problem of electromagnetic self-fields excitation is solved through the slowly varying amplitudes method. We have analyzed the influence of the axial guiding magnetic field on the waves dynamics in a plasma-beam superheterodyne free electron laser with Dopplertron pump in the framework of the cubic nonlinear approximation. We found out that the intensity of electromagnetic radiation increases, when the intensity of the axial guiding magnetic field decreases right up to a critical value. We found the critical value of magnetic field strength, and the saturation levels of electromagnetic waves. We have demonstrated that the investigated free electron laser can be used as a powerful amplifier of electromagnetic radiation in the millimeter wavelength range.