Weak expansive soil reinforced by geogrid, as a protective layer on the slope, is one of the side slope treatments in South-to-North Water Transfer. We performed triaxial consolidated drain tests on expansive soils reinforced by 4 groups of small size unidirectional grids to research their water conductivity. Results show that the water conductivity of geogrid increases under low confining pressure, while reduces with the rise of confining pressure. Geogrid improves the soil strength and works as a water route which accelerates drainage, particularly by vertical reinforcement in the whole section. The results are verified by field tests. It's concluded that the variation of seepage  in expansive soil reinforced with geogrid should be taken into consideration.