Abstract: With the widespread application of flocculants in water and sewage sludge treatment, the potential single or joint toxic effects of their residues in treatment systems are increasingly receiving attention. Polymeric iron sulfate (PFS), polymeric aluminum chloride (PAC) and polyacrylamide (PAM) were used as research objects to explore their potential toxic effects through single toxicity and joint toxicity tests using luminescent bacteria. The results showed that: 1) In the single toxicity test, median effective concentration (EC₅₀) value of PFS, PAC and PAM of luminescent bacteria were 29.41, 71.82 and 1 072.64 mg/L, respectively. The single toxicity effect order was PFS>PAC>PAM. 2) In the joint toxicity of binary flocculants test, PFS-PAM had synergistic effect when mixed with the toxicity ratio of 1∶1, while PAC-PAM had antagonistic effect when mixed with the toxicity ratio of 1∶3 and 3∶1. 3) The concentration addition (CA) and independent action (IA) models were used to predict joint toxicity. The results of PFS-PAM were consistent with the predicted values of CA model, while the results of PFS-PAC and PAC-PAM were consistent with the predicted values of IA model. 4) The three-dimensional deviation response surface results showed that the deviation between experimental observations and CA model (dCA) and IA model predictions (dIA) values changed with the mass concentrations of the flocculants. The CA model was consistent with the results of PFS-PAC, when the mass concentrations of PFS and PAC were high, while for the PAC-PAM system, the prediction of IA model was better than that of CA model. The results indicate that PFS and PAC have higher toxic effect on luminescent bacteria, and the combined use of PFS and PAM may increase the toxic effect and pose environmental risks to microorganisms in water treatment systems.