In recent years, pollution flashovers of transmission lines have occurred in many coastal areas, seriously threatening the safe and stable operation of the power grid. This paper conducts an experimental study on the pollution flashover discharge characteristics of insulators under salt-fog conditions according to the pollution level of insulators in the coastal environment. Taking the disc-shaped suspension glass insulators of transmission lines as the research object, pollution flashover tests of insulators under different salt-fog concentrations and superimposed solid-layer pollution conditions are carried out. The pollution flashover voltage characteristics of the insulators are obtained. It is found that the lower the salt deposit density value is, the greater the influence of salt-fog on the flashover voltage of the insulators. In the combined influence of salt-fog and pollution on the flashover voltage, the influence of salt-fog plays a dominant role. The pollution flashover discharge characteristics of insulators under the condition of salt-fog superimposed with a solid-layer pollution is obtained. The research results can provide a basis for the external insulation design and operation and maintenance of transmission lines in coastal areas of China.