Water quality index (WQI) is widely applied for the assessment of water quality (WQ) according to decision No. 879/QD-TCMT of the Ministry of Natural Resource and Environment in most of the provinces. During the application process, the WQI has revealed many problems, such as inaccurate assessment of water quality status, not addressing locally economic, social and environmental conditions, and easy misleading when using abnormally increased values... To resolve the above problems, the paper presents the steps to adjust the water quality index based on the current water quality components of the old WQI formula, the fuzzy comprehensive evaluation theory with the Entropy rate method. With the surface water quality data series and the status of the water use in the main river canal system in Vinh Long City from 2012 to 2017, the computation steps of coefficients and rates of the assessing water quality components are identified; through the new assessment water quality formula is adjusted and tested.