Abstract: As an indispensable physical support in constructing new energy power systems, gas insulated lines(GIL) utilize internal tri-post insulator. Due to their complex geometric structures and significant DC charge accumulation, these insulators are prone to attracting residual metallic debris. This can cause surface charge distortion and induce surface flashover, severely impacting the safe and reliable operation of DC transmission systems. To address this is sue, we established an experimental platform for tri-post insulator charge measurement and surface flashover tests. The dynamic adsorption behavior of metal particles and dust was observed and analyzed, revealing the charge distribution and surface discharge patterns influenced by different contaminants. The microscopic mechanisms underlying charge accumulation and flashover formation were elucidated. The research findings indicate that metal particles are adsorbed from the top to the bottom of the insulator leg, with minimal adsorption in the midsection, which aligns vertically due to the electric field gradient force. The electric field force and Coulomb force are the decisive factors for cumulative adsorption. Meanwhile, the electrostatic and Van der Waals forces between particles drive explosive diffusion. The specific surface area and pressure-determined static friction are essential for maintaining stable adsorption. Metal particles cause significant negative charge peaks at the top of the insulator and minor positive charge peaks at the bottom, with the middle of the legs being most affected. Linear particles have the strongest impact on charge accumulation and flashover, while flake and spherical particles have a weaker effect. The longer the particle size and the smaller the radius, the lower the flashover voltage under negative polarity. Dust can form large areas of negative charge spots, and the amount of charge accumulation is negatively correlated with surface flashover voltage, which can decrease by up to 32.7%, making discharge highly and hazardous. The sources of surface charge injection include gas ionization, reduced surface conductivity, and equivalent capacitance charging. These research results provide a theoretical foundation for the insulation design and fault protection of GIL tri-post insulator.