Osmotic energy from mixing seawater and river water offers a promising alternative to traditional nonrenewable resources. Harvesting osmotic energy requires the design of ultrathin membranes with high ion selectivity for high ionic conductance. However, lab-scale membranes suffer from high-cost, low mechanical properties, and limited membrane area. Here, we demonstrate the fabrication of large-scale self-standing aramid nanofiber (ANF) membranes with thickness of several micrometers through a simple blade-coating method. The properties of fabricated ANF membranes were investigated in detail, which showed great mechanical chemical stability, high mechanical properties, and surface charge density. The application for osmotic energy conversion was further explored, and the ANF membrane with intact structure gave an output power density of 0.83 W m-2 for 50-fold NaCl. Moreover, the power density can reach up to 7.63 W m-2 when the concentration gradient increased to 500-fold. The ANF energy generator maintained the output capacity for 15 days. This scalable and low-cost ANF membrane provides a promising opportunity to harvest osmotic energy for practical energy plants.