Expansive rock is often met in Middle Route Project of South to North Water Diversion. The expansion of expansive rock has a great influence on the reuse of expansive rock and the exchange soil construction in the area. Experiment on the expansion of expansive rock in the Middle Route of South-North Water Diversion Project were carried out aimed at this problem. The results show that there exists a water content deflection point,if the initial water content is smaller than the water content of deflection point, there are four obvious stages of load-free swelling rate with time such as uniform linear swelling stage, linear rapid swelling stage, slow inclining stage, and steady stage. The expansion speed of the first stage is larger than the second stage. If the initial water content is larger than that of deflection point, there are only three stages except for the first stage in the expansion curve. With the increase of initial water content, the expansion rate decreases. But for different samples, the relationship curves between the initial water content and expansion rate are different. The expansion rate decreases with the increase of burden pressure. When the burden pressure is larger than a certain value, the expansive rock will be compressed instead of expansion. These results provide a basis for the construction in expansive rock area.