Sodium hypochlorite (NaOCl) can effectively react with membrane foulants and thus maintain membrane performance in membrane-based water/wastewater treatment facilities. However, the evaluation of the cleaning efficacy of NaOCl agents remains a major and insufficiently understood issue due to the lack of information on the interactions between NaOCl and foulants. To achieve a better understanding of the cleaning efficacy of NaOCl, we investigated the potential of UV-vis spectral absorbance data to explore the cleaning process of NaOCl for fouled membranes by humic substances (HS). During chlorination, the spectral parameter -DSlope(325-375) (the slope of the log-transformed absorbance spectra in the range of 325-375 nm) increased with reaction time at different pH (i.e., 6, 7 and 9) and Cl-2:DOC ratios (i.e., 6 and 12). More interestingly, the increasing -DS/ope(325-375) can correlate well with the increasing hydrophilicity of HS compounds during chlorination, as well as their decreasing molecular weight. In addition, comparisons of the fouling behavior/permeability of pristine and NaOCl-treated HS samples further suggested that the chlorination at pH 7 and with higher amount of Cl-2 led to a lower UFMI and higher -DSlope(325-375), which was beneficial for the maintenance of membrane permeability. Notably,-DSlope325-375 measured from the forward flushing filtrate decreased at various rates with different chlorine dosages, corroborating its reliability to assess cleaning efficacy. The results of this study demonstrate that the spectral parameter -DSlope325-375 not only conveniently evaluates the cleaning efficacy of NaOCl agents in different pH environments but also helps reveal the NaOCl cleaning mechanism.