The effect of irrigation water quality on unsaturated hydraulic conductivity (k) of undisturbed soil and its mechanism were studied. Results showed that the higher the mineralization degree of irrigation water, the higher the soil hydraulic conductivity became. High sodium-adsorption ratio (SAR) of irrigation water would have an unfavorable effect on soil hydraulic conductivity. The irrigation water with a very low mineralization degree had an irreversible effect on soil hydraulic conductivity, inducing a significant decrease of soil hydraulic conductivity. The effect of irrigation water quality on physical properties of the experimental soil was also studied.