Abstract: Determining the relationship between moisture content and shear strength is of guiding significance for engineering. The direct shear tests are carried out on the unsaturated loess samples with different moisture content and dry densities to study the shear strength of unsaturated loess, and the soil-water characteristic curve of unsaturated soil is determined by the method combining the dew point water potential meter and the filter paper method. Based on the researches on the microstructure of unsaturated soils, the influences of moisture content on the microstructure are analyzed. The test results show that as the moisture content increases, the shear strength of the soils decreases, and the increase in the moisture content reduces the cohesion of the soils and the internal friction angle, but has little effect on the internal friction angle. On this basis, the functional relationship between the moisture content and the shear strength index of unsaturated soils is analyzed, and then by analyzing the functional relationship between the moisture content in the soil-water characteristic curve and the matrix suction, a formula for the shear strength is proposed by regarding the matrix suction as the variable.