| Ulungur Lake, consisting of Buluntuohai Lake and Jili Lake, is the second largest inland lake in Xinjiang and serves important ecological functions. Assessing lake water quality is important for tracking changes and evaluating lake health, and is closely related to the ecological security of the basin. In this study, the current situation and spatial distribution of water quality in Ulungur Lake were evaluated and analyzed using principal component analysis (PCA) and spatial interpolation. The driving factors of water pollution were explored using cluster analysis (CA). The objective was to provide a reference for conserving the water environment and managing Ulungur Lake, and for other typical arid inland lakes. In July and August 2017, a water quality investigation was carried out at eight sampling sites, covering all of Ulungur Lake： (S1) Luotuobozi, (S2) Matou, (S3) Zhonghaizi, (S4) the open area of Buluntuohai Lake, (S5) the Nongshishi fishery administration station, (S6) Xiaohaizi, (S7) the inlet of Jili Lake and (S8), the open area of Jili Lake. Eight water quality parameters were measured including salinity, pH, Secchi disk transparency (SD), total nitrogen (TN), total phosphorus (TP), permanganate index (CODMn), dissolved oxygen (DO) and the 5-day biochemical oxygen demand (BOD5). The water in Ulungur Lake was weakly alkaline, and pollution was dominated by organics, nitrogen and phosphorus. The comprehensive water quality index for the eight sampling sites ranked as follows: Luotuobozi > Xiaohaizi > Matou > Zhonghaizi > Nongshishi fishery administration station > open area of the Buluntuohai Lake> open area of the Jili Lake > inlet of the Jili Lake and spatial differences in water quality were significant. Water quality in Jili Lake was better than in Buluntuohai Lake, water quality was better in open waters than near the shoreline and water quality declined with distance from the inlet . Water pollution at S1 was most serious, and water quality at S7 and S8 in Jili Lake was the best. Based on the spatial distribution of water quality, Ulungur Lake had three distinct areas: Class I included S1, S2, S4 and S5 with the worst water quality in eastern, western and central Buluntuohai Lake; Class II included S3 and S6 in south and north Buluntuohai Lake and the water is becoming more saline; Class III included S7 and S8 in Jili Lake, with much better water quality than in the other two areas. Salinization and inputs of nutrients and organics were the primary threats to water quality in Ulungur Lake. Salinization and declining water quality were attributed primarily to the normalized water cut-off in Ulungur River since 2000. This has resulted in the buildup of salt, exacerbated by organic pollutant inputs from aquaculture in the lake and agricultural non-point source pollution from peripheral areas.