Abstract: Sodium alginate (SA) was used as coagulant aid, combined with the coagulant of polyaluminium chloride (PAC), to explore its coagulation aid efficiency in the coagulation treatment of simulated surface water samples, and the effect of solution pH on coagulation efficiency investigated. Also its coagulation aid mechanism was analyzed by the measurement of Zeta potential of the coagulation effluent. Results showed that the optimal dosage of PAC and SA was 4.00 mg/L and 0.06 mg/L for simulated surface water samples. Under this condition, turbidity and UV₂₅₄ removal rates reached 81.9% and 78.7%, respectively. Coagulation efficiency was influenced dramatically by solution pH, and coagulation aid effect of SA was best when pH was 7.00. In this condition, turbidity and UV₂₅₄ removal rate could reach 84.0% and 83.7%, respectively. Additionally, comparative study results showed that coagulation aid efficiency of SA was equal to that of traditional coagulant aid PAM under the optimal conditions. However, SA dosage was much lower than that of PAM, which was a great cost advantage in water treatment.