Abstract: The experimental research about effect of different clay content on shear strength of loess was carried out to reveal the influence of clay content on shear strength and its microscopic mechanism, and then a scientific basis for engineering practice of loess was provided. Self-made wet sieve device under negative pressure was applied to screen loess samples with different clay content. Different clay content of the loess samples were made to the same dry density, but with different moisture content using static pressure method, and then were taken direct shear test. The results show that with the increase of moisture content, the cohesion of samples with different clay content firstly increased and then decreased, and reached the maximum near 14%, while the friction angle showed a monotonic downward trend. With the increase of clay content, the cohesion showed a rising trend, while the friction angle first decreased and then increased. The effect of clay content on shear strength of loess can be explained by its microstructure.