In this study, the method of using combined cosolvents to tailor the polyamide (PA) active layer during the interfacial polymerization (IP) process was explored for improving the performance of thin-film composite forward osmosis (TFC-FO) membranes. It was found that the TFC-FO membrane prepared by introducing combined cosolvents, that is, simultaneously adding N,N-dimethyl formamide (DMF) into an aqueous phase solution and triphenyl phosphate (TPP) into an organic phase solution, exhibited better performance than adding a single cosolvent, which showed a synergistic effect of combined cosolvents. Compared with the reference membrane without using a cosolvent, the water flux of the TFC-FO membrane using combined cosolvents can be increased by nearly 1.6 times under FO mode and 2.0 times under pressure-retarded osmosis (PRO) mode, whereas its reverse salt flux increased slightly and its salt rejection under reverse osmosis (RO) mode can still reach more than 97%. The PA layer morphology and microstructure of the TFC-FO membranes prepared by adding different cosolvents were thoroughly characterized. The use of combined cosolvents can generate more PA polymer during IP, which leads to the formation of multilayer-like PA morphology and the increase of PA layer thickness.