This paper investigated the discharging mechanism of dry-type transformer by analyzing the outside insulation surface electric-field distribution of a 10kV epoxy resin cast dry-type transformer. Firstly, the 2-D finite element model was established,and on this basis the surface electric-field distributions were analyzed under two operating environments of the dry outside insulation surface and the external insulation surface with wetting layer. The results have indicated that the local electric-field density of external insulation surface rises nearly ten times compared with the dry external surface, which is easy to cause the creeping discharging phenomenon and poses a threat to external insulation. A type of low surface energy coating was used on the transformer to prevent the surface discharging phenomenon, and the electric-field distribution was analyzed when the coating surface was covered with dispersed water drops. Compared with the above results, it is obvious that the surface electric-field density decreases eighty percent, which can effectively increase the life of the epoxy resin cast dry-type transformer.