Vacuum-dielectric interface is the most vulnerable part of vacuum insulation systems where surface electrical breakdown is prone to happen, hence severely restricts the development of advanced electro-vacuum devices with large capacity and its miniaturisation. Generally, a direct and effective way to improve vacuum surface insulation is to alleviate the initiation and development of multipactor phenomena. Inspired by this approach, the authors report a 3D-printed insulation structure designed with a millimetre-scale surface cavity covered by periodic through-pore array via stereolithography, exhibiting remarkable multipactor suppression and flashover threshold improvement, well outperforming the conventional flashover mitigation strategies. Experiments and simulations demonstrate that electrons in the multipactor region pass through-pores and are unlikely to escape from the cavity, hence no longer participate in the above-surface multipactor process, and eventually improve flashover threshold. The proposed approach provides new inspiration for the design of advanced insulators featuring ultra-high electrical withstanding capability and brings up new insight into pertinent industrial applications.