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According to the fact that late rice responses to droughts most sensitively after booting stage, five drought indexes are designed in the tests: the numbers of drought days after booting stage being 7, 10, 13, 16 days and till plant death, respectively. The results show that there exist significant negative correlations between the number of drought days and the number of grains per spike,the number of filled grains per spike. The correlation coefficients are －0.956 and －0.959. Soil moisture decreases rapidly and yield reduction rate rises with the increasing number of drought days. When the number of drought days is up to 18, the plants go dead at a critical soil moisture of 5.57%. The linear and nonlinear regression methods are applied in establishing the equations among the number of drought days, soil moisture, and yield reduction rate. It is concluded that there is slight influence in the first 6 to 7 days of droughts, moderate influence in 8 to 11 days, severe influence over 12 days.