Partial surface dry bands formation and arc generation are the necessary conditions for insulator pollution flashover. It is important to study the dynamic mechanism of dry bands formation and arc generation for insulator pollution flashover prevention. In this paper, XP4-160 porcelain insulator and LXY4-160 glass insulator are taken as the samples, the dry bands formation and arc generation mechanism are studied during flashover process. A flat plate expansion model is established, and three cases of microsegment position division of the expansion model are analyzed. The results show that the continuous high temperature of wet polluted layer provides the conditions for the formation of dry bands, and the thickness of polluted layer shows a linear decreasing trend with time. Different results in different dry bands formation location. The division method of long generation time will aggravate the sudden change of the leakage current. With the insulator surface moisture decreasing due to the Joule effect of leakage current, the surface conductivity of the pollution layer increases at first and then decreases. Once the dry bands is formed, the field strength and temperature there increase sharply, while the conductivity decreases sharply.