WeiChangAn Pill (WCAP) is an effective drug against diarrhoea-predominant irritable bowel syndrome (IBS-D), which has been used in clinical practice for many years. However, the underlying mechanism of WCAP in the treatment of IBS-D remains unclear. Gut microbiota plays an important role in the homeostasis of the colon and its alterations in composition and function are closely related to the pathological evolution of IBS-D. Whether WCAP plays a role in the treatment of IBS-D by regulating the gut microbiota has not yet been revealed, which is worth for further investigation. Therefore, the rat model of IBS-D was established by neonatal maternal separation (NMS) combined with a high-lactose feed, and interfered with WCAP through oral gavage in the present study. After seven days of treatment, gastrointestinal symptoms evaluation and the hematoxylin and eosin staining were performed to determine the effectiveness of WCAP.