Drought is usually thought as being a slowly-developing climate phenomenon. However, flash drought evolves rapidly and reduces time available for impact mitigation. At present, the response characteristics and threshold of flash drought induced disaster for crop is still unclear. In this study, the dry-down experiments were carried out at buckets in different climatic zones. The responses of several leaf photosynthetic parameters to water supply condition were analyzed during flash drought development. The results showed that in the process of flash drought, the soil water content decreased rapidly at first and then slowly. The leaf water content and leaf water potential decreased slowly at first and then rapidly as exponential trends. There were obvious threshold responses of photosynthetic physiological parameters of leaves to the decrease of soil water content, and the thresholds of different physiological parameters were different. The net photosynthetic rate and the maximum carboxylation rate had the identical threshold to available soil water at 0.4. However, the stomatal conductance and transpiration rate had different thresholds to available soil water at 0.5 and 0.4, respectively. The photosynthetic physiological parameters of spring wheat did not respond to the thresholds for leaf water content and leaf water potential significantly. It indicated that the disaster mechanism of flash drought strongly caused by insufficient soil water supply. Furthermore, based on correlation analysis and path analysis, we found that the controlling factors of rapid decrease of net photosynthetic rate of spring wheat leaves were non-stomatal factors, which contrasted with the traditional researcher reflecting the stomatal factor mainly limiting photosynthetic process. This study enriches the cognition of drought impact and provides the scientific basis for drought mitigation.