Abstract: The surface charging and discharging phenomena of spacecraft are prone to occur in severe space environment. Specifically, in the case of coupling strong electromagnetic fields, the discharge damage may occur more easily at a lower charging potential, which seriously threatens the operation of spacecraft. To reveal the mechanism and present suppression method of vacuum surface discharge induced by a strong electromagnetic field, the polyimide (PI) film surface is modified by an ion-exchange method, and a surface discharge platform in vacuum induced by a strong electromagnetic field is set up. Combined with surface trap and secondary electron emission yield (SEEY), the mechanism of surface modification to suppress the vacuum surface discharge of PI film induced by a strong electromagnetic field is systematically analyzed. The results show that numerous shallow traps introduced into the PI surface layer by surface modification account for the significantly reduced surface resistivity and SEEY of PI film. Also, the rates of the surface charge accumulation and dissipation remarkably increase. Meanwhile, the analysis shows that the introduction of shallow trap reduces the SEEY and polarization energy in a DC field of PI, and suppresses the process of gas desorption and ionization and secondary electron multiplication, resulting in the significant improvement of the performance in suppressing the surface discharge induced by a strong electromagnetic field. The research is expected to provide a reference for the protection design of surface discharge induced by strong electromagnetic fields on spacecraft.