A corrugated waveguide filled with an infinitely thin annular plasma is immersed in a strong longitudinal magnetic field. By means of fluid model and linear field theory，the dispersion relation of the electromagnetic wave propagation along the system is derived. The dispersion characteristics and beamwave interactions of lowfrequency plasma modes are analyzed in two cases of various densities of plasma by numerical computation. It is found that the dispersion relation with out electron beam satisfies exactly the Floquet theorem and the frequency of electromagnetic wave could not be confined by plasma oscillation frequency. The low-frequency plasma modes can even be coupled with TM modes as the plasma density i s increased.