Abstract: Overhead transmission line insulators are exposed to nature all year round. Due to the influence of various environmental factors, insulator surface pollution is inevitable. The special name of accumulated pollution depends on its geographical location and the role of the surrounding environment. In line design and operation, the degree of pollution layer will be classified according to light pollution area, medium pollution area and heavy pollution area, The transmission line design in the expansive soil area is often considered as the light pollution area, but with the change of the surrounding environment, the pollution accumulation speed of insulators in the expansive soil area is becoming higher and higher. Based on many years of line operation and maintenance experience, the author found that the insulator pollution layer in the expansive soil area is relatively serious in recent years. Taking the 500 kV balance double line in the expansive soil area in Baise, Guangxi as the column, the dirt adhesion of glass insulators in this area is very strong. It is very difficult to remove the dirt manually during on-line road maintenance, and even can not remove the dirt. In order to clarify the dirt composition, through the extraction, test and analysis of insulator dirt, the author knows that the reason for the high fouling rate of glass insulator in the expansive soil area and the difficulty in cleaning is that the fine particles of montmorillonite, the mineral composition in the expansive soil, are attached to the inside of the umbrella skirt of glass insulator under the action of wind for a long time.