A light metal joint filler is provided. The light metal joint filler is formed by uniformly mixing a solvent with a light metal powder and a silver powder, where a powder particle size of the light metal powder is on a micron scale, and a powder particle size of the silver powder is on a nanometer scale or a submicron scale. A metal joining method of the present disclosure includes: coating a joint of two to-be-joined light metal pieces with the light metal joint filler; and hot pressing the two to-be-joined light metal pieces, so that the silver powder is sintered and bonded with the light metal powder and surfaces of the two to-be-joined light metal pieces, and completing joining of the two to-be-joined light metal pieces after the silver powder is condensed.