A selective etching method with lateral protection function is provided. The steps includes: (a) providing a substrate; (b) forming a plurality of tunnels; (c) forming a lateral strengthening structure at a peripheral wall of the tunnels; (d) removing a bottom portion of the lateral strengthening structure, and a part of the substrate by an etching process so as to form a lower structure and expose an unstrengthened structure; and (f) etching the unstrengthened structure laterally so as to form an upper structure.