To study the influence of SF6/N2 gas mixture on the surface flashover characteristics of basin-type insulators in gas-insulated metal-enclosed switchgear(GIS), electromagnetic-thermal-fluid multi-physical field coupling simulation is conducted to simulate the single-phase basin-type insulators of GIS with different volume fractions of SF6/N2 gas mixture, and the temperatures and the distribution of the surface electric field of the basin-type insulators are calculated for various working conditions. The temperature and surface electric field distribution of the basin-type insulator are calculated. Subsequently, a numerical calculation method for the flashover voltage is constructed based on the volume-time theory, and the carrier generation and multiplication process are comprehensively considered. Finally, the variation of surface flashover probability of basin-type insulators under different voltage amplitudes, gas mixing ratios, and gas pressures is investigated based on the constructed method. The simulation results show that, without changing the original GIS structure, when the pressure is 0.4 MPa and the SF6 content is 50%, the threshold voltage for surface flashover of the insulator is only 0.68 times that under pure SF6 conditions, and the insulator withstand voltage level is comparable to that under pure SF6 conditions by increasing the pressure to 0.74 MPa. The conclusion of the study can provide a reference for the insulation design of eco-friendly GIS.