The water source of the middle route project of South-to-North Water Diversion is faced with inadequate automation of water quality monitoring, low efficiency of pollutant diffusion simulation, and insufficient emergency response capacity for sudden water pollution. Digital transformation is in urgent need. In line with the general requirements of digital twin construction, a digital twin water quality information platform is established by integrating GIS, WebGL, UE4 and other new generation information technologies. The platform comprises key technologies such as static visualization of water quality concentration field, dynamic visualization of water pollutant diffusion, and three-dimensional vertical layered visualization of water quality. The workflow of the platform includes monitoring and prediction, early-warning and alarming, risk simulation, and intelligent emergency responses. The application of the platform has greatly improved the intelligent and refined management level for the middle route project of South-to-North Water Diversion. It also provides a technical reference for modernizing and upgrading the water quality in water source areas of large-scale water diversion projects.