There are still some problems for nanofiber-based forward osmosis (FO) membrane, such as low separation, easy deformation and the weak interfacial strength between active layer and support layer. A novel composite structure as the support layer for FO membrane was designed by introducing a chitosan sub-layer onto hydrolyzed nanofiber mat, in which chitosan filled the porous surface, reinforced nanofiber mat and enhanced interfacial strength between active layer and support layer. The resulting composite FO membranes showed low reverse salt flux (0.93 g/m(2).h) and high water flux (over 60 L/m(2).h) after assembling the chitosan sub-layer under the condition that DI water and 1.5M NaCl aqueous solution were used as feed solution and draw solution, respectively for ten hours. It is worth mentioning that the optimized composite membrane exhibited the lower reverse salt flux compared with nanofiber-based FO membranes reported in literatures while its structural parameter is only 203 mu m. The Young's modulus and tensile strength of composite membranes reached over 90 MPa and 30 MPa, respectively, which were higher than those of most of nanofiber-based and phrase-inversion FO substrates. The results showed a synergistic effect of multiple elements including surface morphology, roughness, internal structure, crystallinity and hydrophilicity on separation and permeability performance as well as internal concentration polarization behavior, which was introduced by chitosan sub-layer between hydrolyzed nanofiber mat and active layer.