Abstract: Propagation characteristic of plane waves passing through infinite dielectric boards is studied using optical methods, and the effect of dielectric windows on microwave propagation is analyzed in the paper. The microwave discharge experiments of polytetrafluoroethylene (PTFE) were performed in vacuum under S-band high-power microwave with a frequency of 2.86 GHz. After a long time of surface flashover, tracks and electrical trees appear on the PTFE material surface. In the 100 ns pulse condition, partial discharge on the dielectric surface has been observed by optical photography, and the partial discharge can still cause electrical trees appear on the material surface. As a comparison, an electrical aging experiment of the polystyrene material was conducted. The experimental results show that the polystyrene material has good resistance to tracking and electrical treeing.