Abstract: The Hurleg Lake is located at Delingha sub-basin，NE Qaidam Basin，in an arid areas（37°14′-37°20′N，96°51′-96°57′E） with 11.3 km-length，10.5 km-width and an average depth of 2.94 m. Its catchments area is about 12 360.0 km2 with two inflow rivers of Bayin River and Baluogen River and one outflow into Tuosu brackish lake. In the area，annual precipitation is only 176.1 mm but the evaporation ranges from 2 000 to 2 500 mm. It is currently the only freshwater lake in the Qaidam Basin. Fluvial and lacustrine sediments were widespread in the area. This study aims to discuss the main reason of keeping fresh water，hydrochemical characteristics，material sources and effect fa- ctors of lake water. Total 30 water sample were collected from the Hurleg Lake in October 2012. All of them were fil- trated by 0.22 μm membrane in the field. Water samples were analyzed for TDS，pH by a pH meter of STARTER 3C and a TDSTester 11+，respectively. The anion concentrations of Cl^-，SO₄^2- and NO₃^- were analyzed with Dionex ICS- 900 and K⁺，Ca²⁺，Na⁺ and Mg²⁺ with ICP-OES. The contents of Na ranges from 115.8 to 189.7 mg·L^-1 with an avera- ge of 131 mg·L^-1，while Ca from 55.1 to 64.9 mg·L^-1 with an average of 59.8 mg·L^-1，Mg from 47.6 to 71.39 mg·L^-1 with an average of 52.1 mg·L^-1，K from 6.5 to 8.8 mg·L^-1 with an average of 7.1 mg·L^-1，Cl^- from 200.3 to 406.3 mg·L^-1 with an average of 300.8 mg·L^-1，and SO₄^2- from 0.08 to 211.5 mg·L^-1 with average of 178 mg·L^-1. Thus，the lake water belongs to Cl^-·SO₄^2-- Na⁺·Ca²⁺ type or [Cl]NaⅢ type with high hardness. The pH value ranges from 8.03 to 8.42 with an average of 8.26. It is almost invariable. The TDS ranges from 785 to 1 150 mg·L^-1 with an average of 848.33 mg·L^-1. The hardness has a range of 322.448 to 392.917 mg·L^-1 with an average of 332.870 mg·L^-1. The val- ues of pH，TDS and hardness increase gradually from west to east and have obvious correlation. The possible reason is the presence and increasing distribution of hydrophytes from east to west. Ternary plots show that the major ion compositions of the lake water and Bayin River are significantly different. The Bayin River is the main river flowing into the lake with HCO₃^--Ca²⁺ type. The contents of SO₄^2-，Mg²⁺ and Na⁺ in the Hurleg Lake are much higher with rel- ative low contents of Ca²⁺ and HCO₃^- than those in Bayin river water. The possible reason is that the Ca²⁺ and HCO₃^- of lake water were precipitated as carbonate minerals during the process of strong evaporation. According to the dis- tribution of all of data in Gibbs Figure，the chemical compositions of the Hurleg Lake water are mainly controlled by rock weathering and evaporation in the whole basin. Because of little rainfall and less human activity，weathering products is the main source of lake components with groundwater input. The Hurleg Lake can keep fresh water may lie in recharge of glacier melt water，groundwater，and the lake itself is an outflow lake

Key words: the Hurleg Lake, hydrochemical, characteristcs of major ions, sources of major ions