Abstract: Based on the helical coordinate system, using the approximate field matching method, we studied the high-frequency characteristics of thicker helical slow wave structure, obtained the dispersion equation, and analyzed the influence of the thickness of the helix slow wave on the phase velocity. The results show that the dispersion curve of the slow-wave structure becomes flat with the increase of helix thickness, and also the dispersion becomes weaker, the bandwidth broadens. The calculation results of our method agree well with the results obtained by CST simulation, which proves that the theoretical method has a higher accuracy than the Hooks model in analysis of the thicker helix.