Analytical theory of electromagnetic field in a helical slow wave structure filled with plasma immersed in a finite axial magnetic field is presented in this paper. By means of helix sheath model and the boundary conditions, the dispersion equation of a magnetized plasma loaded helical slow wave structure is derived. Three particular cases (without plasma, without axial magnetic field and in infinite axial magnetic field) are discussed and the corresponding dispersion equations are deduced. It is found that the simplified results are in accord and with published references.