The electric field distribution characteristics of basin insulators in gas insulated switchgear (GIS) under ultra-fast transient overvoltage (VFTO) are extremely important for the evaluation of its insulation performance. In engineering, the electrostatic field is generally used to replace the transient electric field to evaluate its insulation characteristics. The method which uses electrostatic field is difficult to accurately describe the electric field distribution characteristics of the GIS basin insulator under VFTO. To guide the insulation design of the GIS basin insulator, the transient electric field calculation model of the basin insulator was established in this paper, and the transient electric field distribution characteristics of the basin insulator under the combined action of the quasi-DC component and harmonic component in VFTO were analyzed. In this paper, the high-frequency characteristics of the insulating medium were considered, and the Fourier transform was performed on the measured waveform of 220 kV GIS to obtain the quasi-DC component and harmonic component of VFTO. The transient electric field calculation of the basin insulator under VFTO was realized by COMSOL and MATLAB. The results show that the maximum field strength of each key position on the surface of the basin insulator in the transient electric field calculation is improved compared with the traditional power frequency electrostatic field calculation method. This calculation method can more accurately grasp the electric field distribution characteristics of the GIS basin insulator surface under VFTO. In the insulation design, a 10 % insulation margin should be considered if VFTO peak voltage is directly used as power frequency withstand voltage.