The study was conducted to assess water quality in the canals influenced by the landfill activity using the diversity of phytoplankton. Water samples were used to check the evaluation of phytoplankton for water quality assessment. Four phytoplankton and water samples collected in two periods (period 1 in 4/2018 and period 2 in 10/2018). The parameters for evaluating surface water quality included pH, conductivity (EC), total suspended solids (TSS), biochemical and chemical oxygen demands (BOD, COD), and nutrients (NH₄⁺-N, NO₃^--N, PO₄^3- -P). The results showed that NH₄⁺-N and TSS concentrations in some locations exceeded the B1 column of QCVN 08-MT: 2015/BTNMT. Total 241 species of algae in five phyla (Euglenophyta, Chlorophyta, Euglenophyta, Cyanophyta, Pyrrophyta) dominated by the families of Oscillatoria, Euglena, Phacus species indicating the organically polluted water environment. The Shannon-Wiener diversity indexes (H') in the period 1 and period 2 were 1.51-1.62 and 1.54-1.69, respectively showing that the water quality was at the medium level of pollution. The water quality index (WQI) of the two-sampling periods (53-72) indicated water environment was only suitable for irrigation and similar purposes. The present study showed that the quality of water around the landfill has been polluted, possibly due to the leachate, so it is necessary to have solutions to treat leachate and regularly monitor the environment, to promptly detect and solve pollution problem.