The study was conducted to assess the water quality in the Bac Vam Nao flood control area in 2020. Surface water quality in the Bac Vam Nao area was collected at 7 locations with symbols from VN1-VN7. Water samples were collected three times per year in March, June and September. The water indicators include temperature, pH, total suspended solids (TSS), dissolved oxygen (DO), biological oxygen demand (BOD), chemical oxygen demand (COD), ammonium (N-NH₄⁺), nitrate (N-NO₃^-) and coliform were used to assess water quality in the study area. Water quality is assessed using national technical regulations on surface water quality (QCVN 08-MT:2015/BTNMT, column A1), water quality index (WQI). The main sources of variation and the main indicators affecting the water quality were identified using principal component analysis (PCA). The results show that the water in the flood control area is contaminated with organic matter (low DO, high TSS, BOD and COD), nutrient pollution (P-PO₄^3-, N-NH₄⁺) and microbiological contamination (Coliform). The WQI index (29-88) shows that surface water quality in the flood control area is classified from bad to good, corresponding to the use of navigation and domestic purposes, but needing appropriate treatment. The indicators of temperature and pH have little seasonal variation. DO, TSS, N-NO₃^- in the dry season were higher than that in the rainy season while BOD, COD, N-NH₄⁺, P-PO₄^3-, coliform in the rainy season were higher than that in the dry season. Water quality in the Bac Vam Nao flood control area is influenced by four main sources, PC1-PC4, which explain 84.8% of the variation in water quality. The parameters of temperature, pH, N-NH₄⁺, N-NO₃^-, P-PO₄^3-, DO, and coliform have the main impact on water quality and need to be continuously monitored. The findings show that the water quality in the flood control area has been polluted, it is necessary to have appropriate solutions to improve the water quality.