The annual precipitation data of Jiuwangzhuang and New Tidal Gate, of which the two hydrologic stations are far away from each other, were used to study the wetness-dryness adjustment ability. The hydrologic station of Jiuwangzhuang is located at the source of the Ji Canal and the New Tidal Gate hydrologic station at the estuary. Considering the limitations when using a single Copula function to describe the dependence of variables, two-dimensional mixed Copulas model was established. This model was applied to calculate the synchronous asynchronous encounter probability of rich poor precipitation between the source and estuary of the Ji Canal. The results showed that synchronous encounter probability of poor and rich precipitation were 23.12% and 20.75%, respectively, the asynchronous encounter probability of which was 6.74%. This indicates, without inflow and outflow of the tributary, the wetness-dryness compensation ability is low between the upper and the lower reaches of the Ji Canal.