This paper took five factors including antecedent precipitation index， river roughness， water surface slope， confluence parameterand loss parameter in the calculation of rainfall warning index of flash flood disasters and employed the trend analysis and grey correlation analysismethods to analyze the sensitivities of the factors. The BeigouheWatershed in Henan Province was taken as a case study. The results show that thefactors influencing the rainfall warning index in turn are river roughness， water surface slope， antecedent precipitation index， loss parameter，confluence parameter. And the roughness should be selected as the most sensitive factor in the calculation of the rainfall warning index for flashflood. The study is significant to improve the accuracy of rainfall warning index.