Existing studies have shown that electrical field accelerates the sulfate attack, and thus it can be used to rapidly evaluate the sulfate resistance of cement based materials. However, Na2SO4 was mainly used as the corrosion medium in these studies and there were fewer researches on MgSO4 as attack solution under electrical field. Therefore, in this paper, the mechanism of magnesium sulfate attack under electric field was analyzed by means of corrosion resistance coefficient, X ray diffraction (XRD) and Scanning Electron Microscope (SEM) equipped with Energy Dispersive Spectrometer (EDS). The results showed that sulfate ions migrated into the specimens under the action of electrical field and reacted with portlandite to produce gypsum, which reacted further with the hydration products of C3A to form ettringite. After the hydration products of C3A were consumed, the penetrated sulfate ions continued to react with portlandite. As a result, the main attack product was gypsum, followed by ettringite.