River system is an open system integrating various elements such as economy,society and entironment,in which sub- stance and energy have direct or indirect exchanges through the activities concerned with water and thus there is evolution in dif- ferent ways.According to the dissipative structure theory,the dissipative structure of fiver system was described in this paper and a fiver water quality evolution model was developed based on the theory of information entropy and fuzzy theory,which advances the theoretical and methodological research on water resources systems.This model was applied to the research on the water qual- ity evolution law in Huizhou Reach of the Dongjiang River,and a satisfactory result was obtained.