In order to evaluate the concentration of sulfate ion that would cause sulphoaluminate expansion in concrete, the influence of carbonation on sulfur distribution in hardened cement paste was investigated and the migration characteristic of sulfur under the carbonation was also clarified. The results show that the elemental sulfur distribution in hardened cement paste is relatively uniform before carbonation. However, sulfur element in carbonation area migrates and concentrates on the non carbonation area in the process of carbonation, so the sulfur concentration deceases in carbonated zone and significantly increases in non carbonated zone and the contents of ettringite. The uneven elemental sulfur distribution may cause local sulphoaluminate expansion of concrete.