Both high ionic conductivity and selectivity of a membrane are required for efficient salinity gradient energy conversion. An efficient method to improve energy conversion is to align ionic transport along the membrane thickness to address low ionic conductivity in traditional membranes used for energy harvesting. We fabricated a free-standing covalent organic framework membrane (TpPa-SO3H) with excellent stability and mechanical properties. This membrane with one-dimensional nanochannels and high charge density demonstrated high ionic conductivity and selectivity. Its power density reached up to 5.9 W m−2 by mixing artificial seawater and river water. Based on our results, we attribute the high energy conversion to the high ion conductivity through aligned one-dimensional nanochannels and high ion selectivity via the size of the nanochannel at ≈1 nm in the membrane. This study paves the way for designing covalent organic framework membranes for high salinity gradient energy conversion.