Smart fire protection is important in smart cities. It ensures the safety of people's life and property. Water supply system is the most commonly used in firefighting systems. The smart fire protection facilities based on IoT technologies are developed to monitor the water supply status in real time. It is required to accurately obtain the key data of natural water sources, municipal water supplies and fire pools. They will realize the digital management of fire extinguishing water sources, providing reliable information for firefighting and rescue operations. They will also enhance the availability of fire control information, and enable the on-site fire brigade to quickly develop targeted water supply plans. This Recommendation addresses the monitoring of water system (MWS) for smart fire protection. It specifies the reference model of MWS, as well as requirements and framework of MWS.