Additives have been used to alleviate the membrane fouling in recent years. The excellent properties of rare earth metals provide the possibility for the mitigation of membrane fouling. In this experiment, a group with lanthanum and a blank group had been set to compare their filtration and compression performance of activated sludge, SMP and EPS concentration at different HRT. The result shows that, at HRT of 6 hours, the sludge total resistance of the former was half of that of blank, and the sludge compression coefficient increased by 34.7% compared with that in the blank group, the sludge specific resistance was about 10% of the blank. The SMP and EPS concentration of former was decreased by 20%, and P/C was more than 2 times that of the blank group. Lanthanum chloride alleviated the membrane fouling by improving the filtration and compressibility, reducing the content of pollutants and changing their relative hydrophobicity.