Objective: To determine the efficacy and safety of the Yinmei Kuijie decoction combined with 5-aminosalicylic acid (5-ASA) in treating mildly to moderately active ulcerative colitis (UC) under real-world conditions. Methods: This multicenter, prospective, non-randomized, observational study will be conducted in real-world settings. A total of 204 eligible patients will be consecutively enrolled in the study. Patients in the combination treatment group will receive Yinmei Kuijie decoction in combination with 5-ASA, whereas those in the control group will be treated with 5-ASA alone. The primary endpoint will be a clinical response at week 12, defined as a ≥3 point and ≥30% reduction from baseline in the Mayo total score with ≥1 reduction in rectal bleeding or rectal bleeding score = 0 or 1. Secondary efficacy endpoints at week 12 will include health-related quality of life, mucosal healing, and inflammation indicators. Conclusion The results of this study may provide evidence of the efficacy and safety of Yinmei Kuijie decoction combined with 5-ASA in treating patients with mildly to moderately active UC under real-world principles. The results will provide a basis for further confirmatory studies on the efficacy of Yinmei Kuijie decoction.