Based on the Copula function, a joint distribution model was established to simulate the wetness-dryness encountering probability of precipitation in the flood season, non-flood season, and over a whole year upstream of the Longchuan Station, in the midstream at the Heyuan Station, and downstream of the Boluo Station in the Dongjiang River Basin. With this model, the precipitation wetness-dryness encountering probabilities and conditional probabilities in different scenarios during various periods were calculated. The results show that there are some variabilities in the wetness-dryness encountering probability of precipitation at the three stations. The amount of precipitation was insufficient in the non-flood season, readily causing problems for water diversion. On an annual scale, unfavorable cases for water diversion occurred in most of the study periods, with a probability of 64%. It is suggested that in the planning and design of water resources allocation, reservoirs should be used to rationally allocate and scientifically control water resources, so as to realize the efficient and sustainable utilization of water resources in river basins.