The index of precipitation probability and the grade of dryness/wet-ness based on Gamma distribution are presented in this paper.Sequential analysis on probability index of precipitation (in July,summer and in the year)and of the grade of dryness/wetness in Beijing area suggest that the series of the index and the grade of dryness/wetness based on Gamma distribution are better than the ones based on Normal distribution in describing the climatic states, The distribution of the former is closer to Normal than the latter.The results also show that Gamma distribution is more stable with time in presenting the precipitation.