L₃ skeletal muscle index (L₃ SMI) is an emerging nutritional assessment tool. It has been well documented that L₃ SMI can correctly reflect body skeletal muscle mass, and can be used to evaluate the prognosis of sarcopenia, chronic liver disease and many malignant tumors. Moreover, it can also be used to predict some surgical complications. This review sums up the recent advances of L₃ SMI in evaluating prognosis of diseases.