This is a national prospective, multicenter, observational study. A total of 200 eligible patients will be enrolled from 10 hospitals across the country. Inclusion criteria are as follows: aged ≥ 18 years; diagnosed with moderate-to-severe active ulcerative colitis (UC); disease extent classified as E2 or E3; receiving biologic therapy for the first time; and having a confirmed UC diagnosis for more than 3 months. The follow-up period lasts 30-32 weeks. Six scheduled follow-up visits will be conducted at baseline, weeks 2-4, 6-8, 14-16, 22-24, and 30-32 after enrollment. The study endpoints are defined as: endoscopic mucosal healing (Mayo endoscopic score = 0 for UC), clinical judgment of inadequate response to current biologic therapy (i.e., continued treatment is unlikely to achieve mucosal healing and thus treatment switching or combined medication is required), or completion of the 30-32-week follow-up. The primary objective is to determine the cutoff value and predictive efficacy of the early (Weeks 2-4) reduction in fecal calprotectin (FC) measured by magnetic particle chemiluminescence assay for predicting mucosal healing in moderate-to-severe UC patients initiating biologic therapy.