It is acknowledged that surface charges can accumulate on basin-type insulators of GIL（gas-insulated transmission line） under HVDC voltage

and that the residual voltage can remain on the busbar after cutting off HVDC voltage due to the low decaying rate of surface charge. The existence of residual charges threatens the safe operation of electric apparatus. Therefore

it is of great significance to suppress charge accumulation and improve field distribution of insulator. We established a simulation model of 200 kV basin-type insulator under DC field and defined α as the ratio of surface and volume electric conductivity of the insulating material. The field and charge distributions as well as charge decay time constant with different values of α were calculated for the determination of a reasonable value range of α. The results show that when the value α lies in 1～10 [m]

the DC field will relatively evenly distributed and the surface charge density is lower. The charge decaying time constant is approximately hundreds of seconds with the range of values of α

and the insulator surface is the main way of charge decay. This study may to a certain extent be valuable for the selection of reasonable ratio of surface and volume electric conductivity of insulator for the improvement of DC field distribution and the increase of charge decaying rate.