Bushings play a critical role in DC transmission projects. However, the gas-solid interface within the bushing composite insulation system can accumulate charge under DC voltage, which poses a potential risk for bushing failure. This paper introduced a gas-solid interface charge accumulation model based on an improved saturated current density method. It simulated the potential change process of the ±800 kV valve-side bushing, the transient distribution of the electric field, and the charge distribution at the gas-solid interface. The research findings indicate that at the moment the electric field is established, the distribution is purely capacitive. As time progresses, the electric field transitions to a resistive state, with a notable transition phenomenon observed in the equipotential lines at the gas-solid interface. Initially, the radial field strength of the outermost electrode plate is the highest. When it reaches stability at 3 × 106 s, the maximum radial field strength of 10.6 kV/mm is recorded at the 0th layer electrode plate. The radial electric field distribution of core at 2 h is similar to that at 3 × 106 s, however, the electric field distribution of sheath surface differs significantly. There is a polarity reversal of interface charge at both the core-SF6 interface and the interface between the epoxy glass cylinder and SF6. At the core-SF6 interface, the area near the ground flange side accumulates negative charges, with a peak negative charge density of 13.3 μC/m2 when stable. Conversely, positive charges accumulate near the inlet side of the bushing, with a peak positive charge density of 49.3 μC/m2 when stable.