In recent years, the external insulation discharge and pollution flashover of transmission lines have occurred several times in the vicinity of nickel plant in Yancheng, and they cannot be effectively suppressed after the use of composite insulators. This phenomenon does not happened in the nearby chemical zone. Therefore, pollution samples of insulators are conducted in nickel plants and chemical plants within the jurisdiction of Yancheng power grid for analyzing and testing pollution chemical components, including anion and cation content, insoluble element composition, main compound composition. Then, based on the results of component analysis, a test under mixed pollution is carried out to obtain the flashover characteristics of composite insulators under typical pollution components of different pollution sources. Comparison between pollution composition of near chemical plants and nickel plants indicates that the latter has higher nitrates densities but lower chloride salts density. For one large and two small composite insulators, the flashover gradient under the typical pollution of nickel plants is 5% to 10% lower than that of chemical plants. For the external insulation configuration in heavy industrial dust areas, it is recommended to use one large and one small insulator to improve the electrical strength.