High-voltage and high-power devices are indispensablein spacecraftfor outer space explorations, whose operations require aerospace materialswith adequate vacuum surface insulation performance. Despite persistentattempts to fabricate such materials, current efforts are restrictedto trial-and-error methods and a universal design guideline is missing.The present work proposes to improve the vacuum surface insulationby tailoring the surface trap state density and energy level of themetal oxides with varied bandgaps, using coating on a polyimide (PI)substrate, aiming for a more systematical workflow for the insulationmaterial design. First-principle calculations and trap diagnosticsare employed to evaluate the material properties and reveal the interplaybetween trap states and the flashover threshold, supported by dedicatedanalyses of the flashover voltage, secondary electron emission (SEE)from insulators, and surface charging behaviors. Experimental resultssuggest that the coated PI (i.e., CuO@PI, SrO@PI, MgO@PI, and Al2O3@PI) can effectively increase the trap densityand alter the trap energy levels. Elevated trap density is demonstratedto always yield lower SEE. In addition, increasing shallow trap densityaccelerates surface charge dissipation, which is favorable for improvingsurface insulation. CuO@PI exhibits the most remarkable increase inshallow trap density, and accordingly, the highest flashover voltageis 42.5% higher than that of pristine PI. This study reveals the criticalrole played by surface trap states in flashover mitigation and offersa novel strategy to optimize the surface insulation of materials.