Abstract: Objective To investigate the changes of myeloid dendritic cells(MDC) ratio and interleukin-17 A(IL-17 A) expression in peripheral blood mononuclear cells(PBMC) of patients with severe active ulcerative colitis(UC), and to analyze their relationship with prognosis. Methods Total 90 patients with severe active UC from January 2016 to March 2019 in the Wenzhou Integrated Traditional Chinese and Western Medicine Hospital were divided into good prognosis group(n=49) and poor prognosis group(n=41). MDC in PBMC was detected by flow cytometry, and IL-17 A expression in plasma was detected by enzyme-linked immunosorbent assay(ELISA). Results After treatment, the proportion of MDC in PBMC and the expression of IL-17 A in plasma of the two groups were significantly lower than those before treatment(P ≤ 0.001). The proportion of MDC in PBMC of patients with good prognosis before and after treatment was(0.48±0.15)%,(0.29±0.09)%, which were significantly lower than those of patients with poor prognosis(0.56±0.17)%,(0.44±0.13)%(all P<0.05). The levels of IL-17 A in plasma were(12.97±4.24) ng/L,(9.05±2.96) ng/L, which were significantly lower than those in the poor prognosis group(16.02±5.13) ng/L,(11.88±3.90) ng/L(all P<0.05). The proportion of MDC in PBMC was positively correlated with the level of plasma IL-17 A expression in patients with severe active UC(r=0.692, P<0.001). Diarrhea(≥ 10 times per day) at admission, MDC ratio and IL-17 A were independent risk factors for adverse prognosis of UC patients in severe active stage after treatment(all P<0.05). The AUC of MDC ratio in PBMC combined with plasma IL-17 A expression predicting the adverse prognosis of severe UC patients in active stage was 0.943, the prediction effect was better than single detection. Conclusion Increased MDC ratio and plasma IL-17 A expression in PBMC can be used as potential markers for evaluating the adverse prognosis of severe UC patients in active stage.