Abstract: Irrigation has changed the original environment of loess. In the process of moistening and saturating after irrigation, engineering properties of loess such as microstructure, physical properties, mechanical properties and water-physical properties have more or less been changed, which become important factors inducing loess geo-hazards. Taking Heifangtai of Gansu Province as a study irrigation area, the authors carried out systematic studies of loess, from in-situ tests to laboratory tests, from traditional soil mechanics to mechanics for unsaturated soils, and from microstructures to macroscopic properties. On such a basis, the changes of engineering properties of loess caused by irrigation were revealed. The results show that irrigation has destroyed the original structures of loess with general decrease of both the total amount of voids and the porosity. Physical property indexes consisting of natural water content, natural density and dry density increase, while natural void ratio decreases. Compressibility and collapsibility of loess decrease. The shear strength decreases dramatically. In terms of soil mechanics for unsaturated soils, irrigation causes the increase of water content of loess and decrease of matrix suction. As a result, the strength parameters for unsaturated loess show decreasing trends. In terms of time-related deformation of loess, irrigation causes loess to creep and fail more easily.