Objective To observe the diagnostic value of serum miR-31, miR-155 and chemokine C-C motif ligand 26 (CCL26) in the poor prognosis of ulcerative colitis. Methods 119 patients with ulcerative colitis admitted in our hospital from January 2019 to June 2020 were divided into active phase group (65 cases) and remission phase group (54 cases) according to the disease activity. 45 people for physical examination in our hospital were selected as the control group. The serum miR-31, miR-155 and CCL26 levels were compared in each group, the relation between the levels of serum miR-31, miR-155 and CCL26 and the severity of ulcerative colitis were observed, the univariate and multivariate analysis of factors affecting the poor prognosis of ulcerative colitis were also observed, and the diagnostic value of serum miR-31, miR-155 and CCL26 levels were analyzed for the poor prognosis of ulcerative colitis within 2 years. Results The levels of serum miR-31, miR-155 and CCL26 in the active phase group in patients with ulcerative colitis were significantly higher than those in the remission phase group and the control group (P<0.01), while those in the remission phase group were significantly higher than those in the control group (P<0.01). The serum levels of miR-31, miR-155 and CCL26 increased with the severity of ulcerative colitis (P<0.01). The univariate and multivariate analysis showed that the UCEIS score>6 and the increase of serum miR-31, miR-155 and CCL26 levels were independent risk factors for poor prognosis of ulcerative colitis (P<0.05). The levels of serum miR-31, miR-155 and CCL26 had high diagnostic efficacy in the diagnosis of poor prognosis of ulcerative colitis. The sensitivity of combined detection was 80%, the specificity was 99.4%, and the AUC was 0.963, which was significantly higher than that of single indicator miR-31 (z=2.727, P=0.006), miR-155 (z=3.012, P=0.003) and CCL26 (z=2.091, P=0.036).Conclusion The miR-31, miR-155 and CCL26 are involved in the occurrence and development of ulcerative colitis, and are related to the severity of ulcerative colitis. Combined detection is helpful to improve the diagnostic value of poor prognosis of ulcerative colitis within 2 years