Abstract: In order to study the effect of clay content on the shear strength of expansive soil, this paper conducts quick shear, saturated quick shear, saturated consolidated quick shear and repeated direct shear tests with electric strain controlled direct shear apparatus, residual shear test, and direct shear/residual shear test apparatus on the undisturbed expansive soils at Ci County Hebei of South-to-North Water Diversion. The soils have different clay contents. Studies show that: in the direct shear test, the shear strength of the expansive soil significantly reduces after the soil is saturated, and increases after it is consolidated. The effect of saturation on the strength of the expansive soil of larger clay content is more significant. The cure effect of consolidation on the strength of weak expansive soil of lesser clay content is more significant. As the clay content increases, the cohesion gradually decreases, and internal friction angle first decreases and then increases. The critical value is about 32%.In the repeated direct shear test, the shear strength after peak intensity increases as the clay content increases. The change trend is relatively flat. The residual strength f decreases exponentially as the clay content increases. While the internal friction angle r of the residual strength has a logarithmic relation with clay content. The cohesion cr has an irregular relation with the clay content.