Pure cement and fly ash+cement pastes with replacement rates（by mass） of 10%， 20% and 30% were partially immersed in 10% sodium sulfate solution under environment condition of 20℃ and 60% RH. The results show that the evaporation zone of 30% fly ash+cement paste is severely broken after 60 days immersion， but pure cement paste is damaged until 6 months immersion. The XRD and DTG TG analysis results indicate that more expansive crystals of ettringite and gypsum are generated in evaporation zone of fly ash+cement paste， and more fly ash is added the more ettringite and gypsum crystals are generated. These expansive products are generated due to the pozzolanic activity of fly ash activated by the higher concentration of Na2SO4 solution caused by the accelerating effect of fly ash+cement paste.