Introduction: Ulcerative colitis (UC) is a chronic inflammatory bowel disease (IBD) characterized by a relapsing-remitting course owing to recurrent intestinal inflammation. UC often has symptoms such as intermittent rectal bleeding, diarrhea, and abdominal pain. As the precise etiology of UC has not completely clarified, UC has become a public health challenge worldwide. According to an epidemiological survey, there were about 350,000 new cases of IBD in China from 2005 to 2014. By 2025, the number of IBD patients in China will reach 1.5 million. Traditional Chinese medicine (TCM) has been widely used to treat UC in China, however, it is still challenging to systematically determine the efficacy of in UC. Therefore, this trial aims to evaluate the clinical efficacy and safety of CHM in the treatment of mild active UC patients. Methods: A multi-center, double-blinding, double-dummy, active-controlled, randomized trial will be established. A total of 240 patients in 6 centers with mild active UC (Mayo score is 3-5 points) and TCM syndrome of damp-heat stasis blocking and spleen-qi deficiency will be randomly allocated in the ratio of 1:1 to 2 groups: the experimental group and the control group. The experimental group will receive Hudi enteric-coated capsules (HEC) and enteric-coated mesalazine tablets placebo; the control group will receive enteric-coated mesalazine tablets and HEC placebo. Each group will be treated for 8 weeks. The primary therapeutic outcome: the rate of clinical efficacy and clinical remission at 8 weeks of treatment (last survey point) according to the modified Mayo score. The secondary outcomes: individual symptom score, TCM syndrome score, endoscopic response rate, mucosal healing rate, and quality of life scale score. Outcomes will be assessed at baseline and the end of the trial. Besides, intestinal mucosa, stools and blood biopsies from the mild active UC patients before and after treatment will be collected to reveal the underlying mechanisms. Discussion: The results of this trial will provide compelling evidence of the efficacy and safety of HEC for treatment of mild active UC and preliminarily show the potential mechanism of how HEC acts. Finally, it will widen treatment options for patients with mild active UC.