Tin-based materials have been considered as promising anodes for sodium-ion batteries because of their high theoretical capacity and low-cost. However, their significant volume expansion and low conductivity prohibit their practical applications. A novel CuSn alloy composite with three-dimensional nanoporous structure was prepared by the dealloying method, which can be easily scaled up for mass production. The main component after 24 hours of dealloying is Cu_6.25Sn₅, which displays a bi-continuous structure and the ligament width of 50 nm. The as-prepared nanoporous CuSn alloy composite delivers a capacity of 573.2 mA h g⁻¹. After 100 repeated cycles, the capacity was still 233.2 mA h g⁻¹. This result is much superior to that of pure Sn particles, and the nanoporous structure is well maintained after discharge. The accommodation of volume expansion after discharging, the shortened ion diffusion distance and improved conductivity are expected to facilitate the excellent electrochemical performance. These results demonstrate that the nanoporous CuSn alloy provides unique characteristics for ultrastable sodium-ion battery anodes.