This prospective, open-label, randomized controlled clinical trial aims to evaluate the clinical efficacy and safety of Jingfang Granules in the treatment of diarrhea-predominant irritable bowel syndrome (IBS-D) with cold-dampness encumbering the spleen syndrome, and to explore its potential mechanisms of action. A total of 106 patients with IBS-D of cold-dampness encumbering the spleen syndrome are planned to be enrolled and randomly assigned to the experimental group and the control group, with 53 patients in each group. The treatment will last for 4 weeks, followed by a 12-week follow-up period. The primary outcome measure is the IBS Severity Scoring System (IBS-SSS). Secondary outcomes include the Traditional Chinese Medicine (TCM) syndrome score, the Bristol Stool Form Scale, patient-reported outcome (PRO) scales, the IBS Quality of Life (IBS-QOL) questionnaire, recurrence rate, and others. Exploratory measures, such as immune-inflammatory cytokines and biomarkers related to intestinal mucosal barrier function, will also be observed.