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Crop drought loss sensitivity assessment is fundamental for managing agricultural drought risk. Potted soybean water deficit experiments with different irrigation treatments during two seasons (2015 and 2016) were conducted in China's Huaibei Plain. S-shaped soybean drought loss sensitivity curves at four growth periods were established, and the sensitivity of seed formation to water deficit during different stages was quantitatively compared. The results indicated that the negative effects of water deficit during the flowering-podding and seed filling stages on yield formation were greater than those during the branching and seedling stages. Similarly, aboveground biomass and 1000 seed weight both reached the minimum when soybeans encountered a severe water deficit during the seed filling stage. According to the parameters of the sensitivity curve, the seed yield loss was the greatest when soybeans experienced severe drought stress during the flowering-podding stage. In addition, soybean tolerance to drought during the seedling stage was relatively weak. Moreover, the sensitivity at each growth period varied with water deficit intensity. If there was a mild water defict, soybean yield formation was more sensitive to the deficit when it occurred during the seedling stage. However, if the deficit was relatively serious, yield formation became more sensitive when it happened during the flowering-podding stage. Therefore, it is crucial to strictly control the water deficit intensity during the seedling stage and ensure an adequate water supply during the reproductive growth period, especially during the flowering and pod-enlargement stage, for guaranteeing a high soybean yield.