A new structure of double-layer semi-circular dielectric-loaded surface plasmon polariton (SPP) waveguide is proposed. By using the finite element method (FEM) on the waveguide structure at the telecom wavelength of λ=1550 nm, the transmission characteristics are simulated and analyzed to get the fundamental parameters of this kind of waveguide. The results show that the optimal transmission characteristics of the waveguide propagation can be achieved with the same total area for the waveguide and the refractive index of n1=1.4 and n2=1.473 for the double semi-circular dielectric layers. When the ratio of the inner and outer semi-circular dielectric radii is r1/r2 = 0.5, the maximum propagation length, minimum attenuation coefficient, ideal waveguide mode area and figure of merit can be achieved with the same refractive index parameters, so the optimal transmission results can be obtained.