A helix-type slow wave structure filled partially with plasma is immersed in a strong longitudinal magnetic field.By means of sheath model and linear field theory，the dispersion relation of the slow wave structure is derived. Hybrid modes which are formed at frequencies at which the phase velocities of electromagnetic and electrostatic waves coincide are found.The trend of change for disperstion characteristic,interaction impedance and gain is analyzed in detail in the cases of various densities of plasma and filling radius by numerical computation. Some useful results are obtained on the basis of the discussion.