The trend of muscle wasting in patients with acute severe and moderately severe pancreatitis (AP) remains unclear. This retrospective study aimed to investigate the trend of skeletal muscle area (SMA) changes and its impact on patients with severe and moderately severe AP.Patients diagnosed with AP who had repeated CT scans after intensive care unit (ICU) admission were included. The patients were categorized into moderately severe AP or severe AP groups. The generalized additive mixed model (GAMM) was used to analyze the SMA trajectories.A total of 126 patients were included. The patients in the severe AP group had more rapid muscle wasting during the first 3 weeks following ICU admission. The SMA decreased by 1.1 cm2 (95% CI: 1.3 to 0.8) per day in the severe AP group, while the SMA decreased by 0.5 cm2 (95% CI: 0.6 to 0.4) in the moderately severe AP group in the GAMM model. A larger change in the SMA during the first 10 days after admission was significantly associated with prolonged length of hospital stay (LOS) (β = - 0.205, P = 0.036).Patients with severe AP experienced more muscle wasting during the first 3 weeks after ICU admission. A larger reduction in the SMA was associated with prolonged LOS.Different patterns of muscle wasting were present during the first 3 weeks after ICU admission in moderately severe and severe AP patients. Accordingly, different nutrition and rehabilitation strategies might be employed depending upon the severity of AP.© 2024. The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.