Abstract: After a period of operation, transmission line insulators show non-uniform pollution on their upper and lower surfaces. The pollution has a great impact on the switching impulse flashover voltage of the insulator. In this paper, the wet-pollution method is used to simulate the non-uniform pollution on the upper/lower surface of insulators. The flashover characteristics of clean, uniform, and extremely non-uniform pollution of XP–70 and LXY–70 insulators are analyzed by switching impulse flashover tests in the presence and absence of pre-addition voltage at different salt densities. An ultra-high-speed camera is used to photograph the switching impulse flashover process of XP–70 insulators. The results show that the switching impulse flashover voltage of insulators with extremely non-uniform pollution is negatively correlated with the ratio of upper and lower surface salt density (T/B), and decreases with the increase of equivalent salt density. The pre-addition voltage will also significantly reduce the flashover voltage. In addition, the insulators with extremely non-uniform pollution switching impulse flashover are in the form of sequential flashover along the face segment. The discharge development speed is faster compared to uniform pollution, flashover time is shorter, while arc connection umbrella skirt and steel foot phenomenon exists in the development of trajectory. The results can provide theoretical supports for the design of external insulation of transmission lines in complex and extreme environments, and are an important guideline for the prevention of pollution and flashover of insulators.