[Objective] An experiment focused on the effect of different clay content on the permeability coefficient of loess was carried out, to reveal the influence of clay content on the permeability coefficient and its microscopic mechanism, and then to provide a scientific basis for the permeability of theory and engineering practice of loess.[Methods] Self-made wet sieve device under negative pressure was applied to screen loess samples with different clay content. Laser particle analyzer was carried out to track measurement clay content of the loess soil. Different clay content of the loess samples were made to the natural dry density, but with different moisture content using static pressure method, and then saturated by infiltration experiment.[Results] The clay content of the loess had a great influence on its permeability. For samples with the same moisture content, the permeability of loess reduced gradually as the clay content increased. In addition, permeability coefficient exhibited an exponential and negative relationship with the content of clay as independent variable. The permeability coefficient decreased with the increase of the initial water content for those samples with same clay content. With the increase of clay content, the microstructure of loess samples changed from a dispersed structure to a dense structure. The contact of some particles changed from bracket contact to mosaic contact. The particle connections were connected or bridged into embedded connections. The pores changed from stent macroporous to small ones among clay particles, whereby the permeability of the loess was reduced to some extent.[Conclusion] The responses of the permeability coefficient of loess to clay content and moisture content were similar. Its impact mechanism can be explained by the microstructure.