Abstract: In order to study the synergistic effect of electrons and protons in the combined electron-plus-proton irradiations, S781 white paint samples were irradiated in a simulated space environment with 10 keV electrons, 70 keV protons, and 10 keV electrons plus 70 keV protons, respectively. The irradiation-induced spectral reflectance change and the solar absorptance change were evaluated, and the additive coefficient for the combined irradiation effect was calculated. The results indicate that 10 keV electrons and 70 keV protons have a similar average projected range in S781 white paint and share almost the same damage mechanisms, therefore, the synergistic effect is noticeable in the combined irradiations, which reduces the optical degradation.