The anhydrite rock swells obviously when it's exposed to water, and the non-uniform dynamic distribution of groundwater leads to uneven expansion of surrounding rock.We take the anhydrite rock in Lirang tunnel, Liangzhong highway, as the research object,and set different initial water amount to carry out the anhydrite confined swelling experiment with using the consolidometer, which aims at figuring out the change rules of water absorption, swelling strain and confined swelling stress of the anhydrite rock. The results show that the water absorption rate of anhydrite increases rapidly at first and then slowly. The water absorbed by anhydrite in the process of rapid growth is mainly converted to crystal water, and in the slowly growth process,the free water inside the anhydrite gradually increases.The swelling strain and swelling stress of anhydrite increase with the increase of soaking water. When the soaking water volume is one-fourth of the theoretical water consumption of complete gypsification, it reaches the turning point where the growth rates of swelling strain and stress significantly decrease. The water absorption of anhydrite rock has a time effect, and there is a negative exponent relation between water absorption and time. Based on it, combined with the humidity stress field theory, the anhydrite time-dependent constitutive is obtained.