Gas insulated electrical equipment may suffer from DC superimposed impulse voltage. To further investigate the flashover characteristics of insulator under DC superimposed impact voltage, the effect of DC pre-stressed time, SF6 pressure, voltage polarity and surface roughness on insulator flashover voltage were studied. It was found that when the polarity of the DC voltage and the DC voltage were different, the flashover voltage was reduced due to the enhanced electric field from the surface charge. In contrast, under the same superposition polarity, the flashover voltage is almost the same as under LI alone. The change in flashover voltage increased gradually with increasing DC pre-stress time, and it reached saturation after a certain time. When the SF6 pressure increased, the flashover voltage increased, but the flashover voltage change rate gradually decreased. In the case of superimposed with same polarity, the increase of surface roughness would reduce the flashover voltage, but in the case of reverse polar superposition, it had no obvious effect. This study was helpful to the optimal design of GIS insulation.