Compare the combination of traditional Chinese medicine (Qingre Huashi Quyu) and mesalazine with mesalazine for the treatment of ulcerative colitis.

INTERVENTION: Participants are randomly allocated to one of two groups using SAS9.10 software: 1. Qingrehuashiquyu recipe 200ml, taken orally two times per day and mesalazine 0.25g,taken orally four times per day for 12 weeks 2. Mesalazine 0.25g,taken orally four times per day for 12 weeks Following completion of this 12 week period, there is another 12 week follow‐up period. Clinical symptoms data and samples including blood, stool and colonic mucosa will be collected by clinicians. Blood will be collected by drawing blood of centrifugal vein. Stool will be collected by specimen box. Colonic mucosa will be collected by electronic endoscope. CONDITION: Ulcerative colitis ; Digestive System PRIMARY OUTCOME: Mucosal histology score measured using Geboes index, assessed at baseline and 12 weeks. SECONDARY OUTCOME: ; 1. Mayo Activity Index, assessed at baseline and 12 weeks; 2. Traditional Chinese Medicine syndromes of UC score, assessed at baseline,4,8 and 12weeks; 3. Fecal occult blood test, erythrocyte sedimentation rate, C‐reactive protein, fecal calprotectin at baseline and 12weeks; 4. Detection of intestinal apoptosis at baseline and 12 weeks; 5. Detection of intestinal flora at baseline and 12 weeks; 6. Detection of intestinal nitric oxide at baseline and 12 weeks; 7. Inflammatory bowel disease quality of life scale at baseline and 12 weeks; INCLUSION CRITERIA: 1. The diagnosis of ulcerative colitis is active, and the clinical severity is mild to moderate. 2. Meeting the Traditional Chinese Medicine syndrome of large intestinal damp‐heat criteria. 3. The age is between 18 and 65 years old, and gender is not limited. 4. The patient signs informed consent. The process of obtaining informed consent should be in accordance with GCP regulations.