Abstract: Hydrological forecasting modernization is crucial for addressing climate change, supporting integrated river basin management, safeguarding national security, and enhancing disaster prevention and mitigation capabilities. Following a review of the progress in hydrological forecasting modernization construction within the Changjiang River Basin, and based on the "One-Two-Three-Four" overarching framework for building a modern precipitation-water monitoring and forecasting system, we summarizes the practices in developing an integrated "Space-Air-Ground-Water-Engineering" monitoring network and a prototype digital twin platform for the Changjiang River Basin. We systematically collate the key technological innovations that have propelled forecasting techniques from experience-driven paradigms towards intelligent leaps, highlight their significant benefits in flood control and drought resistance, and explore future development pathways. The research indicates that the Changjiang River hydrological system has established a key technological framework encompassing multi-scale seamless coupled precipitation forecasting, distributed hydrological modeling, forward and reverse feedback prediction, multi-source fused probabilistic flood forecasting, intelligent evolutionary forecast correction, and full-cycle real-time forecasting and operation. Significant advancements have been achieved in monitoring networks, digital platforms, and core technologies through forecasting modernization, yielding substantial benefits in flood and drought management. Notably, the application of the forecasting system during the 2020 big flood in basin-scale successfully prevented the use of the Jingjiang Flood Detention Area, averting economic losses exceeding 10 billion yuan. During the 2022 low water events, application of forecasting system mitigated the impacts on a total population of 13. 85 million, robustly ensuring food and water supply security, thereby validating the system′s effectiveness. The modernization practices of hydrological forecasting in the Changjiang River hold great significance for enhancing basin management capacity and ensuring water security, which can provide crucial technical support and replicable experiences for the intelligent management of mega-scale river basins globally.