The variation of Assimilable Organic Carbon (AOC) during the coagulation of raw surface water and synthetic water was investigated. It was found that the removal rate of AOC concentration during alum coagulation of raw surface water was only about 24.5% at high coagulant dosages. A higher removal rate of AOC in the case of FeCl3 coagulation (about 65.9%) was observed under the same dosages, but AOC-NOX increased from 143 μg L−1 to 473 μg L−1 during FeCl3 coagulation. The results showed a relationship between the solution pH in the coagulated water and the reduction of AOC. The AOC of water decreased efficiently during coagulation in the case of synthetic water at lower coagulant dosages, and the case of coagulation with FeCl3 also resulted in a better removal of AOC. The lowest value of AOC achieved in the case of synthetic water (54 μg L−1), was close to the biostability criteria of drinking water (50 μg L−1). It can be concluded that not only large molecules but also low-molecular weight fractions of organics can be removed during coagulation. The performance of coagulation effect was evaluated based on the following parameters: pH, turbidity, UV absorbance at 254 nm (UV254) and Total Organic Carbon (TOC). The results showed that the coagulation effectiveness has a substantial impact on AOC reduction.