This study demonstrates a sacrificial-layer approach by co-casting, which is the simultaneous casting of two layers, to prepare a polysulfone support (denoted as PSfco) layer with open bottom surface morphology for fabricating thin-film composite forward osmosis (FO) membranes. In the co-casting process, polyetherimide (PEI), used as the sacrificial layer, was co-cast beneath the PSf layer. After the PEI layer was peeled off, PSfco was yielded with an open-bottom structure. Results showed that under the same operating condition, the FO membrane prepared by co-casting (denoted as PSfco-TFC) demonstrated a 10% higher water flux using 0.5 M NaCl draw solution and 30% higher water flux using 4 M NaCl draw solution in the AL-FS mode in comparison to membranes prepared in a single layer casting technique (denoted as PSfs-TFC). The PSfco-TFC exhibits a lower average structural parameter (S, 167 μm) than that of the PSfs-TFC (241 μm), while the water and salt permeability coefficients of both membranes are similar. Results reported here demonstrate that the co-casting technique can be used to fabricate FO membranes with significantly improved performance compared to the conventional approach.