The purpose of this study is to undestand the effect and mechanism of preventing ultrafiltration(UF) membrane fouling by using magnetic ion exchange resin (MIEX®) in terms of fractional compent and molecular weight of NOM. In the experiment, pretreatment decreasing trans-membrane pressure and preventing membrane fouling were studied. The result showed: when the MIEX® dosage reached 800 bed volume (BV), the polymeric aluminium chloride (PAC) dosage could be reduced to 14 mg/L, and the Trans-Membrane Pressure (TMP) could be recovered effectively after hydraulic flush. This pretreatment could remove most of the hydrophobic and hydrophilic charged organics as well as some neutral organics. Foulants of molecular weight (MW) <1000 dalton (Da) and 3000 Da < MW < 100,000 Da organics could be removed by 40.7% and 96.0%, respectively. By reducing pore blocking and membrane cake resistance, the ultrafiltration (UF) membrane reduced the total resistance of membrane. As a result, membrane fouling was alleviated and the water production ratio was enhanced.