[Objective] Three sections of Xiaolangdi Reservoir (Nanshan, Dahengling, and Tongshuling) in the Yellow River were monitored. The present situanion of water quality and potential pollution sources of the reservoir were analyzed in order to provide help for its environmental improvement.[Methods] According to the measured water quality data of the three sections, a principal component analysis was performed to select the main evaluation factors, and the fuzzy evaluation method was used to evaluate the water quality of the reservoir.[Results] Six primary water quality factors were obtained from the principal component analysis: dissolved oxygen (DO), the 5-day biochemical oxygen demand (BOD₅), ammonia nitrogen (NH₃-N), total nitrogen (TN), total phosphorus (TP), and the potassium permanganate index (COD_Mn). The DO concentration and BOD₅ belonged to class I, with values of 3.8—12.8 and 0.9—1.7 mg/L, respectively. The NH₃-N concentration ranged from 0.03 to 0.72 mg/L, which was between classes Ⅰ and Ⅱ. The TN concentration ranged from 2 to 4.88 mg/L, which was worse than the standard of class Ⅴ. The TP concentration and COD_Mn belonged to class Ⅱ, and their values were 0.02—0.05 and 1.8—2.7 mg/L, respectively.[Conclusion] Through a principal component analysis and fuzzy comprehensive evaluation, it was found that the water quality of Xiaolangdi Reservoir basically met the legislative requirements; however, there was a risk of TN and TP pollution.