A new insulation system with inorganic A-B-A insulators was proposed to improvethe surface flashover performance in vacuum. Inorganic A-B-A insulator samples of M0/Al_2O_3cermet-Al_2O_3 ceramic-Mo/Al_2O_3 cermet were prepared, in which the conductivity and permittivityof the Mo/Al_2O_3 cermets were controlled through different amount of metallic molybdenumpowder added. The effects of both conductivity and permittivity of Mo/Al_2O_3 cermets on theDC and impulse surface flashover voltage in vacuum were experimentally investigated. The resultshowed that the DC and impulse surface flashover voltage were improved by 52% and 95%, respectively.For the distribution of electric field, two triple junctions, i.e., vacuum-layer A-cathode(TJ1) and vacuum-layer A-layer B (TJ2) were prepared with the introduction of layer A into theA-B-A insulation system. Based on the electric field distribution obtained via electrostatic fieldsimulation and Maxwell-Wagner three-layer model, the electric field of TJ1 decreases while thatof TJ2 increases with the increase in conductivity and permittivity of layer A under applied DCand impulse voltage, respectively. Therefore, the improvement of surface flashover performance ofA-B-A insulators has been reasonably explained. A new insulation system with inorganic ABA insulators was proposed to improve the surface flashover performance in vacuum. Inorganic ABA insulator samples of M0 / Al_2O_3cermet-Al_2O_3 ceramic-Mo / Al_2O_3 cermet were prepared, in which the conductivity and permittivity of the Mo / Al_2O_3 cermets were controlled The results of both conductivity and permittivity of Mo / Al_2O_3 cermets on the DC and impulse surface were flashover voltage in vacuum were experimentally investigated. The result of the DC and impulse surface flashover voltages were improved by 52% and 95 %, respectively. For the distribution of electric field, two triple junctions, ie, vacuum-layer A-cathode (TJ1) and vacuum-layer A-layer B (TJ2) were prepared with the introduction of layer A into the A- system. Based on the electric field distribution obtained via electrostatic fields simulation and Maxwell-Wagner three-layer model, the electric field of TJ Thus, the improvement of surface flashover performance of A-B-A insulators has been reasonably explained.