Subwavelength periodic structures have many specific properties different from the traditional structures. Based on these properties,new nano-photonics devices can be designed. Using finite difference time domain (FDTD) numerical simulation methods,abnormal transmission phenomenon under TE polarized light excitation is realized by adding dielectric coating in the subwavelength peridic metallic slits. By comparing effects of adding dielectric in different locations such as left or right surface of metal films and metallic slits for extraordinary transmission phenomenon,it is proved that dielectric coatings on the metal films can excite a surface wave which transmits in dielectric waveguides in the metal slits,and results in extraordinary transmission phenomenon.