A novel half rectangular-ring helix slow-wave structure (SWS) is proposed for the design of wide bandwidth and high power traveling-wave tubes. The numerical calculation by 3D electromagnetic simulation software HFSS shows that proper dispersion and coupling impedance can be obtained by reasonably setting the geometrical parameters of the SWS. Meanwhile, compared with the half circular ring helix slow wave structure, slight variation in dispersion and remarkable improvement in coupling impedance have been observed in the numerical calculation of the half rectangular ring helix SWS. The half rectangular-ring helix slow-wave structure has the combined advantages of flatten dispersion, high interaction impedance, easy fabrication and convenience for interaction with sheet beam.