The invention relates to a method of treatment comprising administering a long-acting mutant human fibroblast growth factor to a subject in need thereof. The said long-acting mutant human fibroblast growth factor is mPEG-CH2—;NαH-mFGF21, wherein mFGF21 consists of SEQ ID NO:1, and the said new use consists of a method of treating non-alcoholic steatohepatitis.