The present invention relates to a pharmaceutical composition for the prevention or treatment of non-alcoholic fatty liver disease including a long-acting GLP-1/glucagon receptor dual agonist, and a method for preventing or treatment of non-alcoholic fatty liver disease including administering the composition. The composition of the present invention either has no side effect of weight gain or reduces the side effect of weight gain, which is a side-effect of conventional therapeutic agents for non-alcoholic fatty liver disease, and reduces the amount of administrations of a long-acting GLP-1/glucagon receptor dual agonist, thus greatly improving patient's convenience. In addition, the long-acting GLP-1/glucagon receptor dual agonist of the present invention improves in vivo sustainability and stability.