The water sensitivity of loess refers to the reduction in its physical and mechanical properties when subjected to humidification. The water sensitivity of loess is an important factor affecting the occurrence of water-induced disasters in loess area, and it also determines the resistance ability of loess geological region to disasters, which has great research value. Based on the yield stress of loess, consolidation test and triaxial compression test were used to investigate the variation of yield stress in the process of loess humidification. The mechanical response of loess in the process of humidification was analyzed by the yield stress ratio, and the causes of water sensitivity of loess were explained from the mechanical mechanism. Then, the concept of quantification of water sensitivity was introduced. Based on the changes of mechanical properties of unsaturated and saturated loess, the quantitative index of water sensitivity of relevant loess was proposed and its influencing factors and changing exchange rate were studied. Finally, the empirical formula of Moell-Coulomb strength criterion for water-sensitive loess was presented, which is convenient for the subsequent research and engineering application of water-sensitive loess.