The present invention provides a metal beam having an asymmetrical section and having a damage warning function. The metal beam comprises a body. A sectional shape of a body section has a neutral axis, and has a pressure region and a tension region which are present when subjected to a pure bending moment load. Within an elastic range, each point of the body has a linear relationship with the neutral axis. Two sides of the sectional shape of the body section are asymmetrical with respect to the neutral axis. At a position of the body subjected to the maximum bending moment, a section modulus of the pressure region of the body section is greater than a section modulus of the tension region of the body section. In the body section, after the pressure region yields as a result of stress acting on the pressure region reaching the elastic limit, but before the pressure region deforms plastically, stress acting on the tension region exceeds an elastic limit and therefore the tension region yields and deforms plastically. The tension region entering a plastic deformation stage serves as a warning indicating that a compression or shear failure may occur in the pressure region, and this is the effect achieved by the present invention.