The study was conducted to evaluate the characteristics of black water arising in the rainy season at Lung Ngoc Hoang Nature Reserve, Phung Hiep District, Hau Giang Province. Water samples were collected at the strictly protected area and the ecological rehabilitation area of Lung Ngoc Hoang Nature Reserve. Black water is evaluated using the parameters of temperature, turbidity, DO, TSS, H₂S, EC, BOD, COD, NH₄⁺-N, NO₂^--N, NO₃^--N, PO₄^3--P, SO₄^2-, TN, TP, Fe²⁺, Al³⁺, lignin, and coliform. The values of these parameters are compared with those in QCVN 08-MT: 2015/BTNMT, column A1. The study results showed that black water appears in rainy months from June to October from forest areas plot 6 and plot 85 where are small channels and poor flow. The results of water quality analysis showed that BOD, COD, TSS, TP and TN parameters all exceeded the allowable thresholds, especially DO was very low compared to the standard from 20 to 21 times. Al³⁺ and Fe²⁺ are both higher than the permissible limits. Water is seriously polluted and can affect aquatic life in the study area.