Abstract: Objective To investigate the correlation between the baseline skeletal muscle mass and the severity and prognosis of critical patients in ICU. Methods Seventy-nine patients admitted to ICU for≥48 h were recruited in this study. The （T7） level cross-sectional pectoral muscle area （PMA） upon admission to ICU was measured. The correlation between PMA and 90 d mortality, survival rate to 90 d, ICU-free survival to 28 d, ventilator-free survival to 28 d, ICU mortality and hospital mortality was analyzed. Results After the adjustment of gender, NUTRIC score, Charlson comorbidity index （CCI） and hemoglobin, patients with larger baseline PMA still obtained significantly lower 90 d mortality, higher survival rate to 90 d and ICU-free survival to 28 d, and lower hospital mortality compared with their counterparts with smaller baseline PMA （all P < 0.05）. Conclusions Upon admission to ICU, baseline skeletal muscle mass of critical patients can be utilized to predict the severity of illness, the duration of ventilator use, the length of ICU stay and clinical prognosis of patients.