Dongjiang River basin holds an important political, economic and social status in Guangdong Province. The three biggest reservoirs, namely Xinfengjiang(XFJ), Fengshuba(FSB) and Baipenzhu(BPZ), provide most water for living and production in the basin. Considering the critical requirement of joint operation of multi-reservoirs, this paper constructed 2-dimensional and 3-dimensional joint distributions of inflow of the three biggest reservoirs based on Copula functions, and analyzed their wetness-dryness encountering probability. The results showed:(1) the synchronous wetness-dryness probabilities of any two reservoirs are higher than the asynchronousones,and the synchronous wetness-dryness probabilities in flood seasons are higher than those in non-flood seasons. The synchronous wetness-dryness probabilities of BPZ and XJF, or BPZ and FSB are relatively small, which offers the possibility of wetness-dryness compensation between BPZ and the other two reservoirs;(2) the 3-dimensional joint distribution indicated that the synchronous wetness-dryness probabilities of the three reservoirs in flood season, non-flood season and annual scale are 2.29%, 41.74% and 51.99%, respectively, with the probability of synchronous wetness and dryness much larger than that of synchronous normal ones. The probabilities that FSB and XFJ are able to compensate their lower basin are 29.81% and 23.03%, respectively, while that they cannot do are 32.75% and 22.32%, respectively;(3) By the joint distributions constructed above, we can get the probabilities of any inflow combinations of the three reservoirs, as well as diverse inflow combinations under certain probabilities.