Charges accumulated on the surface of HVDC GIL（gas-insulated transmission line）insulator can lead to the decrease of its flashover voltage and even flashover accident. Therefore

it is of important theoretical significance and engineering value to study the phenomenon of surface charge accumulation under long-term DC voltage and analyze its mechanism in order to propose a more comprehensive model. In this paper

the charge relaxation equation and steady-state charge density equation are introduced based on 3 present accumulation models. The surface charge distribution of a disc insulator is numerically calculated by using the method of micro-element. The results show that charge relaxation equation can describe the relaxation process of surface charge

and that steadystate charge density equation reveals the inevitability of charge accumulation. The latter can be degraded into the present models under different conditions. The distribution of surface charge is closely related to the applied electric field

dielectric constant and conductivity of both gas and solid dielectrics.