Since dams are an important part of the system for river flood control and deriving benefits, their safety is vital. In recent years, the information construction of water conservancy and hydropower has achieved positive results, and the digitalization and intelligence levels of dam safety management have been continuously improved. However, compared with the requirements of high-quality development of water conservancy and hydropower, the intelligence level of the operation is still insufficient, which cannot meet the needs of dam safety supervision in the new stage in terms of engineering monitoring perception, structural performance analysis, and capabilities of monitoring and early warning, etc. Based on the requirements of codes of practice, integrating the concept of digital twins, and taking a Water Conservancy Hub of China as an example, the overall functions and system architecture of the intelligent online monitoring system for gravity dams have been proposed, the key technologies required for online monitoring, such as monitoring data analysis, structural performance simulation, and comprehensive safety evaluation have been introduced, and the future development of technology and the prospects for system promotion and application have been predicted in this paper.