This research focused on the influence of molecular weight (MW) fraction of humic acid (HA) on ultrafiltration membrane (MW cut off of 100 kDa) fouling and further investigated the impact of Al species of polyaluminum chlorides (PACls) on coagulation-ultrafiltration process. Results indicated that MW fraction of 30-50 kDa generated the most severe membrane fouling. On the basis of experiment results in this research and other literatures, we proposed that reversible fouling and irreversible fouling was largely associated with floc properties and residual HA in coagulated water, respectively. This could fully explain the influence of different PACls, PACla (with high monomeric species), PAClb (with high medium polymer species) and PAClc (with slightly higher colloidal or solid species), on membrane fouling. After floc breakage and re-growth, floc size followed the order of PACla > PAClc > PAClb and the sequence of floc fractal dimension was as follows: PAClb > PACla > PAClc. Reversible fouling of PACls was consistent with floc fractal dimension, which was directly correlated with cake layer resistance. In coagulated water, HA with MW more than 50 kDa had similar removal for PACls, whereas HA with MW less than 50 kDa achieved the highest removal efficiency for PAClb, followed by PAClc and PACla.