No.8 Bridge is the site where Yang River flows into Guanting Reservoir. It’s an important guarantee for the water safety of Guanting Reservoir that the water quality meets the standards steadily at No.8 Bridge. Based on the monitoring data from 2006 to 2013, the interannual and annual trends and correlations among DO, NH₃-N and COD of Yang River were analyzed. The factors affecting DO were also discussed. The results showed that DO and COD met the fourth-class standard for surface water. NH₃-N decreased year by year and its standard-reaching rate ran up to 70% since 2009. DO and COD, NH₃-N was negatively correlated, but not significant, indicating that COD and NH₃-N were not important factors affecting DO. DO varied along with seasons, autumn (mean 7.47mg /L) > spring (mean 7.35mg /L) > winter (mean 6.95mg /L) > summer (mean 6.35mg /L). Temperature is an important factor affecting DO. The possible impact on water environment of Guanting Reservoir caused by DO decrease should be paid special attention.