Abstract: Polyetherimide (PEI) is an important insulating material for aerospace applications. It is of great significance to improve surface insulating properties of PEI and to investigate the physical mechanism of surface flashover. In this study, vacuum surface flashover voltage of PEI is effectively improved by 19.25% via nano-TiO₂ composite modification. By investigating the surface morphology, surface outgassing characteristics, and surface charge transport characteristics of PEI before and after the modification, it can be found that gas adsorption capacity is improved with the increase of the mass fraction of TiO₂, while deep trap level and surface resistivity are positively correlated with the flashover voltage. The analysis suggests that the changes in surface gas adsorption capacity is not enough to affect vacuum surface flashover. The essential reason for the improvement of vacuum flashover voltage by nano-TiO₂ composite modification is that the increase of surface deep trap level and surface resistivity hinders the charge injection at the electrode and electron emission of the surface, thus preventing the electron multiplication process. When the electron multiplication process is suppressed, the dielectric needs more energy to promote the escape of dissolved gas inside the dielectric or the decomposition of dielectric, so that the gas collision ionization is sufficient to enhance the electron avalanche, thus leading to the improved flashover voltage.