Gas-insulated metal-enclosed transmission lines (GIL) are one of the important components of DC transmission systems. Under the continuous action of DC voltage, a large charge builds up on the surface of the internal pillar insulator, adsorbing metal particles remaining in the GIL due to various reasons, distorting the surface electric field and inducing surface flashover, affecting the safe and reliable operation of the DC transmission system. Most of the current studies are about the influence of surface charge accumulation or metal particles on surface flashover separately, and there are relatively few studies that take both factors into consideration at the same time. Based on this, this paper built an insulator charge measurement and surface flashover experimental platform, obtained the charge distribution and surface discharge law under different voltage applying times and conducted comparative analysis, and explained the causes of charge accumulation and flashover from a microscopic perspective. Experimental studies have found that metal particle pollution can cause insulator flashover voltage to show a three-stage change of “falling-rising-falling” characteristics. In this process, the particle micro-discharge will not only significantly change the insulator surface charge density and distribution state, and the study also confirmed that there is an obvious positive correlation between the surface charge density and the flashover voltage amplitude. This study can provide a reference for the insulation design and fault protection of GIL post insulators.