This clinical trial aims to evaluate the comparative efficacy and safety of upadacitinib versus infliximab as second-line treatments for acute severe ulcerative colitis, addressing the following key questions: (1) Can upadacitinib as a second-line therapy effectively induce remission in acute severe ulcerative colitis with efficacy non-inferior to infliximab? (2) What adverse reactions may occur with upadacitinib in the treatment of acute severe ulcerative colitis? Researchers will also compare the efficacy of upadacitinib with that of corticosteroids to assess its therapeutic effect. Participants will be assigned to either the upadacitinib group (receiving upadacitinib extended-release tablets 45 mg once daily for 8 weeks, followed by 30 mg once daily) or the infliximab group (receiving an initial dose of 5 mg/kg, with additional doses at week 2, week 6, and every 8 weeks thereafter at the same dose). The treatment duration is 3 months, with outpatient visits for examinations and tests every two weeks. Patients will record their bowel movements and symptoms such as abdominal pain, and undergo colonoscopy, ultrasound examinations, and blood tests at specified time points.