The optical properties of terahertz (THz) waves through the heavily doped semiconductor slits with dielectric layers coated on the slits surfaces have been investigated. The influence of the dielectric coating layer, radiation frequencies, and temperatures on the propagation properties of surface modes have been shown. The results show that the propagation losses have been reduced by coating dielectric layers on the slit surfaces. As the thickness of the dielectric layer increases, the effective indices increase, the propagation losses decrease. The numerical contour results demonstrate that with the increasing of carrier concentration, the effective indices of the surface modes decrease, the propagation losses of surface modes decrease. As temperatures increase, the carrier concentration and dielectric constants of the semidonductor slits increase, resulting in the propagation constants of surface modes decreasing. The numerical results are very useful and important in the fields of THz sensing, spectroscopy and waveguides.