In order to deeply understand river functions, the essence of river function is discussed from the system theory for its sustainable utilization and it is expected to promote theoretical development of river function. The results show that, river function is the ability and effectiveness in the interactional process between river system and environment, it is determined by the structure of river system, achieved through the material flow, energy flow and information flow in the river system, and expressed through meeting demands for river own and external environment. River functional harmony was built by using principles that includes the local interest obedience overall benefit and the lower level needs priority.