Background: Irritable bowel syndrome (IBS) is a chronic, recurring condition, prevalent in the general population. Current medication treatments usually leave patients undertreated. Nowadays, Chinese medicine (CM) is being considered as a promising treatment approach for IBS. However, due to methodological limitations, there is no strong evidence to support CM. Although IBS relapses are common, the relapse assessment has always been neglected in CM study designs. Meanwhile, in clinical practice and studies, it has been found that certain CM formulas can only benefit certain kinds of patients. Discovering what population and illness characteristics likely respond to outcomes may help improve the effectiveness of CM. The aims of this study are to evaluate the efficacy and safety of Tiao-Chang Ke-Min (TCKM) granules for IBS, especially in reducing IBS symptoms' relapse, by a highquality randomized controlled trial and then to optimize the indication of the TCKM granules. Methods/design: This is a parallel-group, randomized, double-blind, placebo-controlled trial embedded with outcome predictive factors. Eligible patients with diarrhea-predominant IBS will be randomized into either a TCKM granule group or a placebo group. Patients from both groups will receive health education. The treatment duration is 4 weeks and the follow-up is 12 weeks. The primary outcome is global improvement measured with adequate relief (AR). The second outcome measures include time until relief, time until first relapse, total relapse times, long-term effectiveness, individual symptoms, IBS-Symptom Severity Score (IBS-SSS), IBS-Quality of Life Questionnaire (IBS-QOL), and Hospital Anxiety and Depression Scale (HADS). Predictive factors associated with patient and illness characteristics have been widely collected. These factors will be embedded in this trial for further identification. Discussion: This trial may provide high-quality evidence on the efficacy and safety of TCKM granules for IBS and a more accurate indication. Importantly, this trial will provide a new research method for improving the therapeutic effects of CM for clinicians and researchers. To address IBS relapse assessment, a series of special definitions of relapse incidents has been made for this trial.