Abstract: In addition to global warming, ocean acidification has become another global environment problem caused by anthropogenic CO2 emissions. During recent years, solar geoengineering (also termed as solar radiation management) has been proposed as a backup plan to counteract global warming by reducing the sunlight reaching the atmosphere and surface. To have a more complete evaluation of the solar geoengineering effect on global climate, it is of great importance to examine the effects of geoengineering on ocean acidification. In this study the UVic Earth System Model was used to simulate the effects of solar geoengineering on sea surface pH and aragonite (a metastable form of calcium carbonate) saturation (Ω). We run the UVic model from pre-industrial to the year 2100 under RCP8.5 scenario, and quantified the effects of individual environment factors on ocean acidification. The simulations show that by the year 2100, relative to pre-industrial levels, global mean sea surface pH would decrease by 0.43 and Ω state will decrease by 1.77. Relative to the RCP8.5 scenario without geoengineering, by the year 2100, solar geoengineering would increase sea surface pH by 0.003, but decrease Ω by 0.16. Relative to the RCP8.5 scenario without geoengineering, geoengineering-induced increase of dissolved inorganic carbon would decrease pH and Ω, but the increase of alkalinity would increase pH and Ω. Geoengineering-induced decrease of temperature would increase pH but decrease Ω. The net effect of solar geoengineering on pH and Ω is small. Our study indicates that solar geoengineering could cool the earth but fails to mitigate ocean acidification.