The research aims to evaluate the efficacy and safety of that mesalazine with hydrocortisone sodium succinate for the induction of clinical remission during a 4-week double-blind treatment period in active UC (define as total Mayo score of greater than or equal to 4 and less than or equal to 10). A total of 528 patients will be randomly divided into three group, one will receive mesalazine 4g with hydrocortisone sodium succinate 100mg enema, and the other two group will respectively to receive mesalazine 4g and hydrocortisone sodium 100mg one times daily for 4 weeks. The end of the study for every patient is the improvement of main symptoms.The primary endpoint is the clinical remission after 2 and 4 weeks double-blind treatment, defined on the basis of a total Mayo score ≤ 2 points, with no subscore > 1 point. The secondary endpoint are endoscopic mucosal healing at week 2 and 4 of double-blind period, defined as an absolute subscore for endoscopy portion of the Mayo score of 0 point and the change from baseline in Quality of Life at week 4 of double-blind period based on the IBDQ.