Based on the Maxwell equations, the general equation of the slow-wave structure filled with plasma in the finite magnetic field is derived. The dispersion equation and interaction impedance expression of the disk-loaded waveguide filled with plasma in the strong longitudinal magnetic field are studied. The result shows that the frequency of the TM01 mode upshifts and interaction impedance increases as the density of the plasma increases. When a periodic structure is loaded with plasma, the spectrum consists of abundant TG modes (Trivelpiece-Gould modes). As the plasma density increases to a certain degree, the TM01 mode of the disk-loaded waveguide overlaps the TG mode and these two modes will couple with each other and form the new hybrid modes G1, G2. If the relativistic Traveling-Wave Tube (TWT) works on the hybrid mode, there will be new working mechanism.