Abstract: According to the fundamental concept of crop water stress coefficient and some related equations in FAO56, a new formula computing crop water stress coefficient was deduced considered the temporal change of critical soil water content. This formula can fully express the effects of soil water availability and crop potential evapotranspiration on crop water stress. This formula and another power function formula were applied synchronously to field water balance analysis of Xiaohe Irrigation Station, Shanxi Province. Both of them can simulate the variation of soil water content fairly well and acquire good soil water balance results. And the calibrated parameters from the two models are nearly the same. Compared with the power function formula model, parameters of FAO model are less and can be obtained more easily through optimization. Especially, the FAO model can obtain the fluctuation curve of the critical soil water content which reflects the probability of crop water stress during the process of soil water depletion. The critical soil water is low at the initial growing stage of winter wheat, very high at the middle stage and moderate at the end stage. From greening-harvest stage, the maximum value of the critical soil water content of 0～1 m layer is 295 mm, the minimum value is 215 mm and the average value is 247 mm. This curve can serve as a reference to agricultural water management.