In order to carry out studies of the relationships between the onset of Plum rains and the rainfall amount during Plum rains period in the Taihu Lake basin, the authors have firstly selected optimal univariate marginal distribution, and then established the joint distribution of two variables based on the Copula function, and at last, analyzed the encounter probability and the conditional distribution probability between the onset of Plum rains and the rainfall amount during Plum rains period under different circumstances. The analysis results show that there are significant differences in the encounter probability between different onsets of Plum rains and different rainfall amounts during Plum rains period, and that in general the maximum probability is encountered between the normal onset of Plum rains and the average rainfall amount during Plum rains period. There is high probability for the conditional distribution probability that the average rainfall amount during Plum rains period appears in the different onsets of Plum rains. But the probability of wet Plum rains in the early onset of Plum rains is 0.417, and meanwhile, the probability of dry Plum rains in the late onset of Plum rains is 0.374. So close attention should be paid to the changes of flood when appearing in the early onset of Plum rains, and the changes of the drought when appearing in the late onset of Plum rains. This study can provide a reference for taking measures of flood and drought control and water resources regulation for the Taihu Lake basin as soon as possible.