The invention discloses a rectangular waveguide covered with a dielectric layer in the technical field of communication cables. A waveguide structurally comprises the rectangular waveguide and the dielectric layer, wherein the dielectric layer is arranged on the rectangular waveguide; the wall of the rectangular waveguide is provided with a slot array formed by a plurality of slots at uniform intervals along the axis direction. The dielectric layer is added on the rectangular slot waveguide, and the electric field intensity of a working area on the surface of the rectangular slot waveguide is obviously improved; the divergence degree of an electromagnetic field is reduced; meanwhile, proper thickness and dielectric constant of the dielectric layer are selected, and the radiation intensity of a working area of a rectangular slot waveguide cable can be controlled. The rectangular waveguide can be applied to a communications based train control (CBTC) railway mobile communication system, the near field intensity on the upper side of the rectangular slot waveguide is improved, the space convergence of an electric field is improved, and the system has the best working condition.