Critical flux is an important concept in fouling control for membrane-based processes. Operation below the critical flux can maintain the membrane flux and reduce the maintenance cost which is associated with membrane cleaning and replacement in forward osmosis (FO) processes. In this research, the effects of foulant type, binding ions concentration, and cross-flow velocity on the critical flux in FO processes were investigated using a draw solution concentration stepping method. The results showed that the draw solution concentration stepping was feasible for the critical flux determination in FO processes. The thin-film composite (TFC) membranes exhibited a low critical flux for sodium alginate (SA) fouling with a value of 29.32 L/(m2?h), then followed nano-silica (SiO2) with a critical flux value of 32.17 L/(m2?h) and humic acid (HA) of 46.35 L/(m2?h). This indicated that the critical flux behavior in FO processes was dependent on the properties of both the membrane and foulants, including the membrane surface roughness, intermolecular adhesion of foulants, and the interaction between foulants and membrane. The atomic force microscopy (AFM) results revealed the deposition of foulants onto the ridge-and-valley structure of the membrane surface, leading to the deviation of water flux from the baseline with the increasing draw solution concentration. As the Ca2+ concentration increased from 0 mmol/L to 10 mmol/L, the critical flux for alginate fouling dramatically decreased from 29.22 L/(m2?h) to 9.48 L/(m2?h), which can be attributed to the interaction between alginate and membrane as well as the intermolecular aggregation of alginate and the interaction by Ca2+ binding. Moreover, the critical flux for SA?Ca2+ complexes fouling increased from 9.48 L/(m2?h) to 31.59 L/(m2?h) with the cross-flow velocity ranging from 5 cm/s to 15 cm/s, which indicated the improvement of the solution turbulence can enhance the critical flux, thereby expanding the operating ranges of flux.