Abstract: Water level monitoring is crucial for the fields of hydrology, hydraulic engineering, and disaster prevention and control. Traditional methods of monitoring water levels have several drawbacks, including high costs and limited coverage. The Global Navigation Satellite System (GNSS) serves multifaceted functions, including navigation, positioning, and timing. Additionally, it can harness its reflection signal to extract characteristic information from the reflecting surface, a technique known as GNSS reflection signal remote sensing (GNSS-R). In recent years, GNSS-R technology has emerged as a novel approach for water level monitoring, offering advantages such as cost-effectiveness, all-weather operability, and high temporal and spatial resolution. This paper provides a comprehensive review of the current state of research in GNSS-R water level monitoring. It discusses the influencing factors of different methodologies and identifies challenges encountered in the application process. Furthermore, it proposes future development trends and anticipates its potential application in China’s water conservancy industry.