Ultrafiltration membrane was used to filter the Yangtze River water by powder activated carbon (PAC) pretreated, such as Tai Lake water by coagulation-sedimentation pretreatment. The changes of organic polar content were compared in each process. Combined with the scanning electron microscope(SEM) photographs of the membrane, the mechanism of slowing down membrane fouling by PAC and coagulation-sedimentation pretreatment was studied. The results showed that hydrophobic organic compounds were the main factors of membrane fouling, while hydrophilic organics had less impact. When the dosing quantity of PAC filtered by ultrafiltration membrane was 20 mg/L to pre-treat the water samples of Yangtze River, and that of polyaluminium chloride was 25 mg/L for the pretreatment of Tai Lake water samples, the flux could be recovered effectively after backwashing compared with filtering the two raw water directly. The cake layer formed on the surface of the membrane was looser after pretreatment, and could be discharged easily by periodic backwashing, The cake layer could prevent hydrophobic organic matters deposit on the surface of membrane directly, which will slow down the membrane fouling and improve the chemical safety of the membrane effluent.