Abstract: Using the topographic data of 1918 and 6 remote sensing images in different times from 1965 to 2006, 7 urban river distribution maps of downtown Shanghai were derived in this research. The spatio-temporal evolution of urban river system and its impacts on the environment were analyzed based on GIS and spatial analysis techniques. The main results include: (1) From 1918 to 2006, the quantity of rivers in downtown Shanghai has decreases from 1 018 to 184, correspondingly the areas of them have shrunk from 37.68 km2 to 15.26 km2. (2) The meander-cutoff and obstruction of river are two major manners of the river system reduction. And the trend of urban river system diminishment is from the center to the surrounding. (3) The primary driven force of urban river system evolution is human activity, such as constructions of buildings and roads, and cropland degradations. (4) The evolution of urban river system disrupts the system’s water storage capacity, and weakens its urban heat island effect. Based on the analysis, several suggestions were proposed for the future planning.