Background: Qingchang Suppository, a formula used for more than 30 years in Longhua Hospital, has shown satisfactory clinical effects on Ulcerative Colitis (UC). However, its therapeutic mechanism has not been fully elucidated. Purpose: The study aims to investigate the effects of Qingchang Suppository powder (QCSP) and its ingredients by regulating the IL-17A signaling pathway which plays an important role in the development of UC. Methods: HPLC was used to analyze the main ingredients (Gallic acid, Indigo, Indirubin) in QCSP. HT-29 cells were induced by rhIL-17A and TNF-α, and IL-17A related protein expressions were determined by western blot. BALB/C mice were induced by 4% Dextran Sodium sulfate (DSS). The effects of QCSP and its ingredients were evaluated by measuring weight loss, disease activity index (DAI), colon length, histological analysis. Western blot was used for analysis of IL-17A and MAPK related proteins p-ERK, p-JNK, p-P38. Quantitative reverse transcription polymerase chain reaction (q-PCR) was used to detect the expression of IL-17A, HSP90 and ACT1 in colon tissue. Cytokines such as IL-17A, IL-1β, IFN-γ and TNF-α were determinated by enzyme-linked immunosorbent assay (ELISA). Results: QCSP had good therapeutic effect on DSS-induced colitis in mice. QCSP significantly relieved weight loss, restored colon length, repaired colon lesions, reduced histological scores and DAI, decreased TNF-α, IL-1β, IL-17 and IFN-γ contents, significantly suppressed the gene expressions of IL-17A, ACT1 and HSP90, and up-regulated the expressions of tight junction proteins like ZO-1 and Occludin. IL-17A pathway related proteins such as IL-17A, IL-17RA, HSP90, MAPKs, P-iκbα and iNOS were significantly increased in vitro and in vivo. Conclusions: This paper reveals that QCSP inhibited the IL-17A signaling pathway in HT-29 cells and DSS induced mice, presenting a new mechanism of QCS on treating UC.