Objective:To investigate the prevalence of malnutrition in hospitalized patients with ulcerative colitis (UC) using the Global Leadership Initiative on Malnutrition (GLIM) criteria, compare the body composition, clinical indicators, and occurrence of poor UC outcomes between the malnutrition group and the non-malnutrition group, and analyze the risk factors of malnutrition.Methods:A total of 104 UC patients hospitalized in the Affiliated Hospital of Qingdao University during 2019 to 2021 and with relevant data collected through UC-sarcopenia Prospective Study were included. Data including nutritional risk screening 2002 (NRS 2002) scores, diagnosis of malnutrition per GLIM criteria, body composition data, clinical data and occurrence of poor outcomes were extracted. All patients were screened for nutritional risk using NRS 2002 at admission, and those at nutritional risk were further evaluated for malnutrition diagnosis per GLIM criteria. The prevalence of malnutrition in UC inpatients, the differences in indicators between malnutrition and non-malnutrition groups, and the risk factors of malnutrition were analyzed.Results:Among the enrolled UC patients , 35 (33.7%) were at nutritional risk, and 30 (28.8%) were diagnosed as malnutrition by GLIM criteria. Compared with the non-malnutrition group, the body composition indexes, including body fat mass, protein, skeletal muscle mass, segmental muscle mass, body water, waist circumference, arm circumference, visceral fat area, basal metabolic rate and body cell mass, all decreased to some extent in the malnutrition group ( P < 0.05). Prealbumin and albumin levels were lower ( P < 0.01). The modified Mayo endoscopic score (MMES), C-reactive protein level and erythrocyte sedimentation rate were higher ( P < 0.01). The rates of re-admission and surgery due to disease activity at 90 days were higher ( P < 0.05). High MMES ( OR =1.534, P = 0.044) and low albumin level ( OR =0.781, P = 0.013) were risk factors for malnutrition in UC patients. Conclusions:GLIM criteria is suitable for the diagnosis of malnutrition in hospitalized UC patients, and malnutrition patients are more likely to experience poor outcomes such as short-term re-admission and surgery. UC patients with higher MMES and lower albumin level are more likely to suffer from malnutrition.