Abstract: Experimental research and engineering practice proved that the result of stabilizing soft soil with waste gypsum and cement is much better than that with cement only.During the hydration of cement waste gypsum, in addition to calcium silicate hydrate produced by the cement, there is a lot of ettringite produced by the reaction of cement with gypsum . The function of the calcium silicate hydrate is cementing loose soil particles together, and that of the ettringite is filling pores by expansion. It is the concentration of CaO and OH - in the pore water of stabilized soil that determines the suitability of cement gypsum and the strength enhancement effect of it.