This study was to assess the effect of powdered activated carbon (PAC) pre-adsorption time on ultrafiltration performance for surface water treatment. Experimental results demonstrated that membrane fouling could be mitigated by extending the pre-adsorption time. The molecular weight (MW) distribution of water samples was determined by liquid chromatography – organic carbon detector (LC-OCD) and results showed that the mechanism of PAC controlling fouling was attributed to a decrease in the low molecular weight (LMW) fraction of raw water via extending the pre-adsorption time. Fouling indexes (FIs) were used to evaluate membrane fouling potential and the results showed that polysaccharides (PS) and proteins (PN) were greatly responsible for membrane fouling. Therefore, prolonging the PAC pre-adsorption time and decreasing the PS and PN content in raw water are the main measurement for the alleviation of membrane fouling when PAC is used as the pretreatment in waterworks.