The pressure retarded osmosis (PRO) process is a novel technology which generates green electrical energy via semi-permeable membranes. However, a major challenge in the PRO system is the lack of suitable membranes with satisfactory power density (i.e., the power output per unit membrane area). In this study, we have successfully fabricated a novel thin-film composite (TFC) PRO membrane consisting of a tiered structure of polyetherimide (PEI) nanofibrous support reinforced by functionalized multi-walled carbon nanotubes (f-CNTs) and an ultrathin polyamide-based selective top skin layer. The tiered support was made by a fine and a coarse PEI nanofiber layers. The thin finer fiber reinforced with well dispersed f-CNTs has been found to increase mechanical stability of the polyamide selective layer, allowing the support to withstand high hydraulic pressure in the PRO system. Our optimized membrane can endure a trans-membrane pressure up to 24 bar and generate a peak power density as high as 17.3 W/m² at 16.9 bar using synthetic seawater brine (1.0 M NaCl) as the draw solution against deionized (DI) water. In addition, the long term PRO result shows that this membrane can generate a stable power density of 15.0±0.5 W/m² for a test period of 10 h. This demonstrates that our membrane holds great potential to be used in the PRO process.