Very fast transient overvoltages (VFTOs) generated during the routine operations of disconnector switch in GIS make great threat to the insulation of power equipment such as GIS. It is very important to obtain the characteristics of VFTO generated in GIS substation, and then analyze the insulation characteristics of GIS under VFTO, including SF6 gas gap breakdown characteristics and insulator flashover characteristics, so as to provide a basis for insulation coordination under VFTO. In the speech, the development process of VFTO simulation generator was introduced first. Then, the insulation characteristics of GIS under VFTO were obtained by using the VFTO simulation generator. Finally, the steep impulse test technology was proposed for GIS to improve the effectiveness of insulation defect detection.