The infiltration of irrigation water on Heifangtai loess plateau raises the groundwater level and saturates the deep soil all the time. Lixiviation caused by underground water takes away much salt and changes chemical components in pore water at the same time, which affects the strength of loess. In this study, the shear strength and physicochemical characteristics of loess are investigated using ring shear apparatus, laser particle size (LPS) analyzer, Zeta probe (potential measuring apparatus), inductively coupled plasma emission spectrometer (ICP-OES), ion chromatograph. The test results show that the curve of strength (internal friction angle)－soaking time is in a “spoon” shape. Based on the results of physical and chemical tests, mechanism of strength weakening of loess soaked in water is discussed. The cements (soluble salt) among loess particles dissolve in water rapidly, breaking the microstructure and making the internal friction angle decrease. At the same time, ion concentration in the pore water increases, and the ions exchanged with outer layer of clay particles, leading to a decrease in the thickness of adsorbed water of clay particle so as to make the internal friction angle increase slightly. With the increase of soaking time, gypsum dissolves in sodium chloride solution and coarse particles are dispersed into clay particles further, then the total thickness of electrical double layer increases, so the internal friction angle decreases slightly.