A kind of hybrid surface plasmonic waveguide working in the terahertz frequency range and based on un-flat substrate structure was proposed. The relative permittivity of InSb changed with the working frequency was investigated. The mode properties of the proposed structure and two other un-flat substrate structures, such as the effective mode area, propagation length, figure of merit and energy ratio, were investigated in detail by varying with the working frequency and geometrical parameters. The results show that the proposed structure can reach the same propagation length with the traditional structure, while the mode area of the proposed structure is smaller by more than an order of magnitude compared with that of the traditional hybrid waveguide, which indicates stronger mode confinement. The proposed waveguide satisfies the requirement of the application in high density circuits in the terahertz range.