Properties of optical propagation as well as interference in the sub-wavelength metallic waveguide based on the extraordinary transmission are investigated by the Finite Difference Time Domain(FDTD) method. And several parameters on the standing wave as well as the coupling of two standing waves are studied, which is useful to understand the properties of optical propagation and the extraordinary transmission in the sub-wavelength metallic waveguide. It shows that TE light field is hard to propagate efficiently in the sub-wavelength metallic waveguide as the waveguide width is much smaller than the half wavelength. The propagating distance of the TE light field will be larger with the increasing of the waveguide width and can propagate generally when the waveguide width is about or larger than the half wavelength. There is a cut-off wavelength in the waveguide and it is linearly proportional to the absorption coefficient of the metal. In addition, due to the extraordinary transmission, it generates the standing wave in the sub-wavelength metallic waveguide.