The surface charge accumulation of basin insulators is one of the important factors restricting the development of HVDC GIL, and the heat transfer inside the GIL would aggravate the surface charge accumulation. In this paper, an electrical-thermal coupling simulation model of charge accumulation was established. The time-varying temperature distribution inside the GIL under different external ambient temperature and gas pressure was simulated. The surface charge accumulation characteristics under constant and time-varying external ambient temperature were studied, and its effect on the surface charge accumulation was also analyzed. The results show that when the external ambient temperature is a constant value, the external temperature increases every 10℃, the steady-state temperature on the insulator surface increases by more than 9.2%, and the steady-state surface charge density of insulator increases by more than 17.3%. When the external ambient temperature changes with time, the temperature on the insulator surface eventually fluctuates with time around a stable value after continuously rising for a period of time, and the surface charge density is approximately equal to that under the average external ambient temperature. In addition, the surface temperature and charge density of the insulator decrease with the increase of gas pressure. The research results are expected to provide reference for the design and operation of DC GIL, which can improve the safety and stability of DC GIL operation.