Abstract: The present invention relates to a human Lefty A protein variant with improved productivity and stability, a fusion protein comprising the protein variant, and a composition for preventing and/or treating neuromuscular disease comprising the protein variant or the fusion protein. According to the present invention, a human Lefty A protein variant and a fusion protein comprising the variant are constructed, which have better stability than naturally occurring human Lefty A protein, and thus are expressed at high levels and produced in high yield in animal cells. In addition, administration of the constructed human Lefty A protein variant or fusion protein can restore the nerve and motor functions of nerve disease model animals. Accordingly, the use of the human Lefty A protein variant or fusion protein can effectively prevent or treat various nerve diseases and muscle diseases.