Abstract Filtration behaviors, membrane fouling and cleaning were investigated in a forward osmosis (FO) membrane system used to post-treat the effluent of a membrane bioreactor (MBR) fed with landfill leachate (LFL). In short-term tests, it was observed that the water flux with the membrane active layer facing the draw solution (AL-DS mode) was lower than that with the membrane active layer facing the feed solution (AL-FS mode) for LFL treatment. Mathematical models could well simulate the flux evolution of AL-FS mode while the flux of AL-DS deviated from the modeling curve, suggesting that fouling could be rapidly developed within 1h filtration for AL-DS mode. During long-term filtration, about 98.6% of COD, 96.6% of TP, and 76.9% of ammonium were rejected by the FO system. A decrease of water flux was also observed with an increase in operation time. Confocal laser scanning microscopy and Fourier transform infrared spectroscopy confirmed the existence of polysaccharides and proteins in the fouling layer. Inorganic fouling was mainly caused by Ca, Na, Mg, K, Si, Fe and Al. It was also found that the effect of cake enhanced concentration polarization played an important role during long-term operation. About 88.9% of the permeate flux was recovered after hydraulic cleaning while it reached 98.9% of the initial flux after chemical cleaning, indicating that chemical cleaning was needed to eliminate irreversible fouling and to recover membrane permeability during long-term operation.