Abstract: Groundwater quality evaluation is the premise of the protecting and rationally exploiting of groundwater resources.In order to accurately reflect the groundwater quality in Delingha basin,12 general chemical indexes and 4 toxicological indexes were selected using a combination evaluation method of worst index judgment method and improved Nemerow index method, which was based on 66 groups of Quaternary groundwater quality test data in Delingha basin, Qinghai Province.The results show that:①the overall quality of Quaternary groundwater in Delingha basin is good.Samples from grade I-Ⅲ groundwater accounts for 74.24% of the total,and grade IV accounts for 10.61%,grade V for 15.15%. ②The quality of confined (artesian) water in Delingha basin is better than that of phreatic water, and the groundwater quality in the alluvial lacustrine plain areas is better than that in other areas. ③Among the 16 indexes evaluated, 5 indexes,which are sulfate, total hardness, natrium, TDS and chloride, exceed the standard level and the rate of excess is greater than 10%.The major transnormal items are the general chemical indexes in the groundwater,while the 3-nitrogen and toxicological indexes are less exceeding.④The quality of groundwater in this area is more controlled by the original geological and natural environment and less affected by human activities.The specific landform, geological structure and sedimentary environment controlled the formation of aquifer and the cyclic evolution of groundwater,thereby controlling the overall quality of groundwater, while the arid and rainless climate affects the quality of shallow groundwater.