Abstract: In order to study the direct shear strength of unsaturated cohesive soil, we used the non-expansive soil and weak-expansive soil of two dry densities to conduct direct shear tests that contain 8 levels of overburden load including low pressure state. The test shows that with the increase of overburden press, the maximum shear stress and the corresponding displacement increase gradually. The cohesive force measured under low overburden press is smaller than that measured under high press; the overburden press exerts a larger influence on the cohesive force of non-expansive soil than that of expansive soil; the internal friction angle measured under low press is larger than that measured under high press. Besides, we analyzed the influence of contact area decrease of upper and lower box on the measurement of direct shear tests, and the influence is validated to be slight. It is suggested that we should adopt shear strength parameters measured under low overburden pressure when shallow failure of expansive soil slope is analyzed.