In order to reveal the impacts of forests on evaporation and runoff in river basin, the coupled model SSiB4T/TRIFFID was employed to conduct numerical simulations for responses of evaporation and runoff on the dynamic changes of vegetation cover over two tributaries of the Yangtze River. The mechanism of forests effecting evaporation and runoff was analyzed. The results showed that in the process of long-term vegetation succession, the forests in the Suomo River Basin located in the upper reaches influence the evaporation and runoff mainly through changing the soil evaporation and canopy interception. The hydrological effects are related to the ability of canopy interception and soil evaporation. The forests in Qingyijiang River Basin located in the down reaches increase evaporation and reduce runoff significantly through their relatively strong transpiration.