The paper studied the annual runoff wetness-dryness encountering of Hongjiadu which was the upper reaches of Wujiang River representative station and Goupitan which was the downstream of representatives station for providing the basis of cascade reservoir schedule. It used the copula function to construct the model of Joint probability distribution between Hongjiadu and Goupitan for the analysis of frequency of the synchronous and asynchronous of the runoff wetness-dryness. The results show that the frequency of synchronism was greater than that of asynchronous. The abundance of the same frequency is slightly larger than the dry frequency, with the frequency of wetness-dryness minimum. The Copula function can effectively analyze the runoff encountering problems of the upstream and downstream in Wujiang river and provide reference for the plan and management of river basin.