Geopolymer has been attracted world attention as a potentially revolutionary material that is one of the ideal substitutes of portland cement. Fundamental studies on geopolymer are increasing rapidly because of its potential commercial applications. However, little work has been done on its erosion resistance of sulfate. In this paper, slag and metakaolin as source materials, different modules of water-glass as activator were used to prepare slag-based geopolymer which was then prepared to make slag-based geopolymer concrete. Linear expansion rate and mass loss of concrete were used as indicators to explore different erosion rules of slag-based geopolymer concrete which was long-term immersed in the 5% ( by mass ) Na2SO4 solution. Comparative experiments were also done between slag-based geopolymer concrete and ordinary cement concrete. The results showed that geopolymer concrete had excellent sulfate resistance performance, linear expansion rate and mass loss rate of geopolymer were 0.024% and 0.58% respectively at the soaking age of 49d as the water glass modulus was 1.4 and the water-glass content was 4%. It was also found that the anti-sulfate ability of geopolymer concrete is much better than that of ordinary concrete with the same strength grade.