Abstract: Three methods were applied to water quality assessment for the Suyahu reservoir, including the single factor water quality identification index, comprehensive pollution index and organic pollution index methods. The assessment was based on water quality data from 2009 to 2015 in addition to water quality requirements. The results suggested that TP and TN were the main pollution factors and the effect of COD, BOD₅ and oil class were secondly in the reservoir, it was the combination of the factors that caused the reservoir water quality only four months qualified, sixty months in bad class water and annual water quality always in pollution; From the perspective of comprehensive pollution, there was only fifteen months that water quality qualified, the rest period in pollution with twenty-eight months in severe polluted, which made its water quality of annual and most of water season was in pollution even severe pollution; From the perspective of organic pollution, there was three months in mild organic pollution for the reservoir and organic pollution index was less than 2.0 in other months, which resulted in its water quality of annual and water season changed between normal and better.

Key words: Suyahu reservoir, single factor water quality identification index, comprehensive pollution index, organic pollution index, assessment