Molybdenum disulfide (MoS2) has shown large promise in harvesting osmotic energy. However, the current investigations generally focus on proof-of-concept nanoscale single-pore devices with a semiconductor phase structure. Exploration of the application viability of MoS2 in a more robust macroscopic-scale two-dimensional (2D) nanofluidic membrane and acquisition of fundamentals of how the phase structure influences the power generation process are highly demanded. Here, we demonstrate that robust and stable composite membranes made up of 2D metallic MoS2 can act as high-performance osmotic power generators. Both experiment and simulation reveal that the higher electron density of metallic MoS2 increases the affinity of cations to the surface, which renders the system excellent ion selectivity and high ionic flux and greatly promotes transmembrane ion diffusion. When natural river water and seawater are mixed, the power density can achieve about 6.7 W m-2. This work shows the great potential of metallic MoS2 in nanofluidic energy devices.