A tape helix show wave structure is filled with uniform plasma.A dielectric is loaded inside the region between helix and a conducting waveguide.The system is immersed in a strong longitudinal magnetic field.By means of helix tape model and linear field theory,the field equations and the boundary conditions in plasma region and dielectric region are given.On the basis of the field analysis,the dispersion relation of the slow wave structure is derived and some discussions are given.