Chau Pha Lake is one of the large reservoirs providing water for domestic, irrigation and production of people in Phu My town, Ba Ria Vung Tau province. This study was conducted to analyze the parameters of pH, temperature, DO, BOD5, COD, N-NH4+, P-PO43-, TSS, turbidity, total Coliform on 16 samples in 2020 and the water quality index calculation WQI. The analysis results show that the water quality of Chau Pha lake in the rainy season is quite good, all parameters reach the limit value A2 according to QCVN 08:2015/BTNMT; however, in the dry season, the values of parameters DO, BOD5, P-PO43, TSS exceed the limit value A2 but not significantly; WQI values fluctuate at a fairly high level from 71 to 92, which is correlated with turbidity, COD, and BOD5 parameters. Thus, the average WQI of the lake reaches 85.4 points, corresponding to level 2 (green) - water used for domestic water supply purposes but needs appropriate treatment measures.