Composite flocculants have been extensively studied and applied in recent years in order to improve the water treatment efficiency. In this study, a new composite flocculant prepared by polyaluminum chloride (PAC) and sodium alginate (SA) was used to treat dye and humic acid water in the coagulation–ultrafiltration process. The subsequent effects of PAC/SA on ultrafiltration membrane fouling were investigated by calculating the Modified Fouling Index (MFI). The results showed that the application of PAC/SA could not only restrict the membrane fouling but also improve the removal efficiency of the coagulation–ultrafiltration process. MFI of PAC/SA was the lowest, followed by PAC and the raw water for coagulated effluents filtered by ultrafiltration membrane. For example, MFI of PAC/SA was 0.40 s mL−2 for reactive blue KGL (denoted as RB-KGL) treatment, while that of PAC was 2.26 s mL−2. The removal efficiencies were improved as coagulation was used as pretreatment of ultrafiltration membrane. And PAC/SA could form the higher removal efficiency than PAC, especially for RB-KGL. The color removal efficiency of PAC/SA was 96.36% for RB-KGL treated by coagulation–ultrafiltration process, which was higher than that of PAC (85.62%).