Mechanosensitive (MS) channels are extensively studied membrane protein for maintaining intracellular homeostasis through translocating solutes and ions across the membrane,but its mechanisms of channel gating and ion selectivity are largely unknown.Here,we identified the Ynal channel as the Na+/K+ cation-selective MS channel and solved its structure at 3.8 A by cryoEM single-particle method.Ynal exhibits low conductance among the family of MS channels in E.coli,and shares a similar overall heptamer structure fold with previously studied MscS channels.By combining structural based mutagenesis,quantum mechanical and electrophysiological characterizations,we revealed that ion selective filter formed by seven hydrophobic methionine (YnalMet158) in the transmembrane pore determined ion selectivity,and both ion selectivity and gating of Ynal channel were affected by accompanying anions in solution.Further quantum simulation and functional validation support that the distinct binding energies with various anions to YnalMet158 facilitate Na+/K+ pass through,which was defined as binding-block mechanism.Our structural and functional studies provided a new perspective for understanding the mechanism of how MS channels select ions driven by mechanical force.