The water salinization in the Beidagang Reservoir is the issue to the safety and the utilization of water resources in the Tianjin coastal area. To study the water salinization in the Beidagang Reservoir, we sampled the river water, reservoir water and groundwater in the wet season (August) and dry season (December) 2015, respectively. The analysis was made in the stable hydrogen and oxygen isotopes (δD，δ18O) and hydrochemistry. The results show that the water evaporation in the wet season was stronger than that in the dry season, causing the stable hydrogen and oxygen isotopes enriched in the wet season. The river water and reservoir water was Na-Cl (SO4), however, the groundwater was Na-HCO3 Cl. The water at the Beidagang Reservoir was controlled by evaporation crystallization processes. Furthermore, the closer to the Bohai coast line, the high - er the electrical conductivity and the more enriched isotopic composition of water. To the Beidagang Reservoir, the salt content of water increased in the flow path, while the strong evaporation accelerated the salt accumulation, resulting in the water salinization. Further analysis on the factors and contribution to salinization should be carried out to provide a theoretical basis for reservoir management and operation.