Beitang Reservoir is planed as a regulating and accident standby reservoir in Tianjin city in the project of the South-to-North Water Diversion Engineering. To determine the security of water quality, total ion amount and Cl^- in reservoir water and sediment and in soil around reservoir were detected from July to December of 2005. Effects of water supply, sediment release, rainfall, evaporation and groundwater level on water quality of reservoir were analyzed too. In this period, Cl^- concentration varied from 302 mg/L to 409 mg/L, which exceeded the maximum of drinking water source standard. The mean content of total ion amount in reservoir sediment from 0-100cm was 0.253% , and chloride content was 0.048% , which account for only 1/8 of total ion amount and 1/18. 3 of chloride content in soil around Beitang Reservoir. Therefore, water storage in reservoir led to release of salt from sediment. According to the 1.3 g/m² release intensity of chloride every day, water quality of Beitang Reservoir can satisfy to the standard of drinking water source when water level keeps above 5.6m. Total ion amount and Cl^- concentration in groundwater around reservoir were 59.63g/L and 26.17g/L respectively, which were much higher than that in reservoir water. As long as the running water level of reservoir keeps above 5.6m, the reservoir would be not affected by groundwater. By the reasonable control, Beitang Reservoir is feasible to work as a regulating and accident standby reservoir in Tianjin city in the near future.