Abstract: To investigate the volume expansive of cement paste subjected to sulfate attack, based on microporous mechanics, this paper established the mechanics model of representative volume element (RVE) composed by cement paste matrix and ettringite crystal. From microcosmic scale, the distribution of expansive strain within RVE under a certain amount of ettringite formation is analyzed. Based on homogenization method, the expansive strain of RVE on microcosmic scale is converted to the expansive equivalent strain of RVE locative point on macroscopic scale. And the variation regularity of expansive equivalent strain with the amount of ettringite formation is studied. The results show that on microcosmic scale, the isotropic strain of ettringite sphere with RVE of 0.1 porosity is tensile strain, the radial strain of cement matrix spherical shell is compressive strain, but the hoop strain is tensile strain. On macroscopic scale, for RVE of 0.1 porosity, the radial expansive equivalent strain is compressive strain, the hoop expansive equivalent strain is tensile strain, and they both increase with the larger amount of ettringite formation.