The performance of a hydrolysis acidification + anoxic/aerobic activated sludge (HA+A/O) process under the load shock of 2,4-dichlorophenol (2,4-DCP) was investigated. Results indicated that 2,4-DCP concentration was below 100 mg/L responding to less than 50 mg/L chemical oxygen demand (COD) of HA+A/O effluent. However, further increasing 2,4-DCP concentration from 200 to 500 mg/L subsequently led to the significant reductions of COD removal. The protein-like and humic-like fluorescence intensities of HAHA+A/OA/O effluent increased gradually with 2,4-dichlorophenol concentration from 0.5 to 500 mg/L. 2,4-DCP concentration of HAHA+A/OA/O effluent gradually increased from 1.3 to 475.4 mg/L responding to 2,4-DCP removal rate from 98.8 to 4.5%. Meanwhile, the degradation intermediate products (4-chlorophenol, 4-CP; phenol) of 2,4-DCP were monitored continuously. 4-CP and phenol concentrations reached the highest values of 7.89 mg/L and 5.73 mg/L, respectively with 2,4-DCP concentration 200 mg/L. Based on chloride element conservation equation between influent and effluent, the dichlorination efficiencies decreased from 16.2 to 0.02% with 2,4-DCP concentration from 0.5 to 500 mg/L. In addition, volatile fatty acids (VAF) and oxygen uptake rate (OUR) inhibition rates of HAHA+A/OA/O increased gradually under the load shock of 2,4-dichlorophenol. When 2,4-DCP exceeded 200 mg/L, VAF and OUR inhibited rates were over 50%, respectively. Ultimately, the prediction modelling between biological toxicity and COD removal was established using regression method, which exhibited a good fit (r(2) = 0.78). This study could help to predict the effect of 2,4-DCP on the performance of the HAHA+A/OA/O process. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.