A method for fabricating a flexible free-standing ultrathin (nano) or thin protein membrane that includes (1) keeping a dilute metal (Cd, Cu or Zn) nitrate or chloride solution under neutral or weak basic pH to spontaneously form metal (Cd, Cu or Zn) hydroxide nanostrands; (2) mixing the metal (Cd, Cu or Zn) hydroxide nanostrands and protein solution to obtain composite nanofibers made of protein and the metal (Cd, Cu or Zn) hydroxide nanostrands; (3) filtering the obtained dispersion of composite nanofibers on a filter; (4) cross-linking the proteins contained in the composite nanofibers by a bifunctional cross-linker; and (5) removing the metal (Cd, Cu or Zn) hydroxide nanostrands.