Bao Dinh River is currently receiving a large number of pollutants due to population activities and economic development in the area. Water quality tends to worsen during the dry season months due to stagnant water from the operation of the sluices to prevent salinity from the Tien and Vam Co rivers. This study focuses on assessing surface water quality at flowing through Bao Dinh river, Tan An City using water quality index (WQI) in the period 2019–2021. WQI index in this study was calculated according to the formula with 3 groups of parameters: Group I: pH; group IV: DO, BOD5, COD, N–NO2–, N–NO3–, NH4+ P–PO43– and group V: Coliform based on Decision No. 1460/QD–TCMT dated November 12, 2019 in Vietnam. The results show that surface water quality at monitoring sites in residential areas is more polluted than in other areas because these areas have higher levels of BOD5, COD and Coliforms. Surface water quality in the dry season tends to be better than surface water quality in the rainy season.