Abstract: The efficient extraction of sodium (Na⁺) and lithium (Li⁺) from seawater and salt lakes is increasingly demanding due to their great application value in chemical industries. However, coexisting cations such as divalent calcium (Ca²⁺) and magnesium (Mg²⁺) ions are at the subnanometer scale in diameter, similar to target monovalent ions, making ion separation a great challenge. Here, we propose a simple and fast secondary growth method for the preparation of MIL-53(Al)-NH₂ membranes on the surface of anodic aluminum oxide. Such membranes contain angstrom-scale (~7 Å) channels for the entrance of small monovalent ions and water molecules, endowing the selectivities for monovalent cations over divalent cations and water over salt molecules. The resulting high-connectivity MIL-53(Al)-NH₂ membranes exhibit excellent ion separation performance (a selectivity of 121.42 for Na⁺/Ca²⁺ and 93.81 for Li⁺/Mg²⁺) and desalination performance (a water/salt selectivity of up to 5196). This work highlights metal-organic framework membranes as potential candidates for realizing ion separation and desalination in liquid treatment.