Mechanisms that electrons are directly accelerated by the laser-plasma interaction in non-resonant cases are studied.First, by use of a linearly polarized Gaussian laser beam, a three-dimensional model is presented to demonstrate that the frequency and the amplitude of electron oscillations can be significantly modulated by the transverse ponderomotive force, within the confinement of an underdense plasma channel. On the one hand, the transverse ponderomotive force can felicitously make electrons to experie...