ObjectiveTo investigate the value of third lumbar skeletal muscle mass index （L3-SMI） in predicting the long-term prognosis of patients with acute-on-chronic liver failure （ACLF）， and to provide a useful tool for prognostic scoring of ACLF patients. MethodsA retrospective analysis was performed for the data of 126 patients who underwent abdominal computed tomography （CT） scanning and were diagnosed with ACLF in Shanxi Bethune Hospital from December 2017 to December 2021， including clinical indicators， biochemical parameters， and model for end-stage liver disease （MELD） score， and L3-SMI was calculated. The independent-samples t test was used for comparison of normally distributed continuous data between groups， and the Mann-Whitney U test was used for comparison of non-normally distributed continuous data between groups； the chi-square test was used for comparison of categorical data between groups. The receiver operating characteristic （ROC） curve was used to assess the diagnostic value of L3-SMI and other variables （MELD score and Child-Pugh score）， and the DeLong test was used for comparison of the area under the ROC curve （AUC）. ResultsAmong the 126 patients enrolled， 44 （35%） died within 2 years and 82 （65%） survived. Compared with the survival group， the death group had significantly higher age， incidence rate of ascites， international normalized ratio， MELD score， and Child-Pugh score （all P<0.05） and a significantly lower value of L3-SMI ［38.40 （35.95‍ ‍—‍ ‍46.29） cm²/m² vs 44.19 （40.20‍ ‍—‍ ‍48.58） cm²/m²， Z=-2.855， P=0.004］. L3-SMI had an AUC of 0.720 in predicting 2-year mortality in ACLF patients， with a sensitivity of 63.6% and a specificity of 80.5%， and a combination of L3-SMI， MELD score， and Child-Pugh score had a significantly better AUC than a combination of MELD score and Child-Pugh score in predicting 2-year mortality （0.809 vs 0.757， Z=2.015， P<0.05）. ConclusionL3-SMI has a high predictive value for the prognosis of ACLF patients， and the combination of L3-SMI、MELD score and Child-Pugh score has a higher predictive value for ACLF patients， and the inclusion of L3-SMI or sarcopenia in the conventional prognostic scores of ACLF patients may increase the ability to predict disease progression.