A canonical formulation is given of the motion of the equilibrium electrons in a reversed-guide-field free-electron laser. It is found that the helical orbits of electrons are always stable. By generating Poincare surface-of-section maps, the influences of the self-fields on the phase orbits of electrons are investigated. The results show that although the current of the electron beam is high up to 6000A, the phase orbits do not become chaotic for MIT's experimental parameters. Compared with the case of a conventional positive-guidefield, our results indicate that using a reversed guide field can apparently prevent the quality of the electron beam from degeneracy.