The present invention relates to a composition comprising Aster sphathulifolius Maxim extract for preventing, improving, or treating muscular diseases and for improving muscular functions. The Aster sphathulifolius Maxim extract according to the present invention exhibits effects of inhibiting the expression of muscle protein breakdown-related factors and increasing the expression of muscle protein synthesis-related factors, as well as alleviating the reduction of muscle tissue weight and cross-sectional area of muscle fibers, and thus can be useful for preventing, improving, or treating muscular diseases and improving muscular functions.