Pre-deposition technology is a promising method for alleviating membrane fouling, and in situ hydrolytic flocs have strong adsorption ability. In comparison with iron salts, less corrosiveness is induced by aluminum salts. Here, flocs formed by aluminum salts were pre-deposited onto the surface of an ultrafiltration (UF) membrane, aimed at investigating the antifouling performance with humic acid (HA) or bovine serum albumin (BSA). The results showed that Al-based flocs were well distributed onto the membrane surface, and a relatively dense and homogeneous deposition layer was formed. Both HA and BSA could largely be adsorbed/rejected by the pre-deposited layer during filtration. In comparison to results for solution pH of 8.0, the floc layer was much denser with a larger surface specific area and smaller average pore diameter at pH 6.0. Due to the existence of Al-13 species, a more positively charged floc layer occurred at lower pH values, leading to stronger adsorption abilities toward the negatively charged HA and BSA molecules. The floc layer significantly reduced the possibility of HA and BSA molecules reaching the UF membrane, which helped alleviate membrane fouling under lower pH conditions. Furthermore, the results for effluent with pH between 6.0 and 8.0 showed that the removal efficiency of HA molecules with small molecular weight (< 3 kDa) was also higher, especially at pH 6.0.