In this paper, the effect of different water content and constant-amplitude dry wet cycle on the strength of Fuyang expansive soil was studied by using the conventional indoor direct shear test. The experimental results were explained and the fitting curve was obtained , and the development of cracks in the process of dry and wet cycle was analyzed. The experimental results show: the water content and the wet-dry cycle have important effects on the strength of expansive soil. The shear strength index c, φ has a good linear relationship with the water content. The strength index has a good exponential relationship with circle times. The strength of the soil is gradually close to the residual strength after several dry wet cycles. The development of the fracture is the main reason for the decrease of the strength of the expansive soil under the wet dry cycle. The fractures destroy the structure of the soil and make the cohesive force decrease. So that the frictional force is reduced. The opening of the fissure causes the bite friction force to decrease.