Abstract: An estuarine reservoir in northern China is characterized by intermittent inflows, continuous outflow, and dynamic salt release from sediments. We developed a quantitative model coupling water quantity and salinity, and using the proposed Muguandao reservoir as an example, examined factors that influence salinity, specifically those that cause it to exceed the standard. We also explore methods to maintain standard salinity levels in a mixed estuarine reservoir. The results showed that the cumulative effects of millimeter-scale evaporation on salinity in reservoir water are noticeable when the water level decreases. Under adverse hydrological conditions, salinity in a mixed estuarine reservoir, affected by salt release from sediments, evaporation, and water outflow, tends to exceed the standard. In contrast with the constant water level conditions, salinity increased with increasing water outflow when water levels decreased. Therefore, decreasing daily outflow could help maintain salinity standards. To ensure water quality safety, the daily available water as determined from the integration of water quantity and salinity, should factor in the management of an estuarine reservoir.