Objective To evaluate the efficacy and safety of Tongxiening granule （TXNG） in the treatment of diarrhea-predominant irritable bowel syndrome （IBS） （stagnation of the liver-qi attacking the spleen）. Methods In a prospective, randomized, placebo-controlled, double-blind clinical trial, 60 patients with diarrhea-predominant IBS were randomly divided into the TXNG group （TXNG, 5.0g, 3 times daily; n =30） and the placebo group （placebo, 5. 0g, 3 times daily; n =30）. The treatment was administered for 3 weeks, and the follow-up was conducted for 4 weeks.Results （1）Abdominal pain： The cure rates were 57. 7% vs. 16. 0% （ by per-protocol analysis, PP） and 31.0% vs. 7.1% （ by intention-to-treat analysis （ITT） ; and the overall improvement rates were 92.3% vs. 44.0% （PP） and 82.7% vs. 39.3% （ITT） in the TXNG and the placebo groups respectively （ P 〈0. 05）. （2） Diarrhea ： the cure rates were 46. 2% vs. 20. 0% （PP） and 41.4% vs. 17. 9% （ITT） , and the overall improvement rates were 96. 2% vs 48. 0% （PP） and 86. 2% vs 42.9% （ITT） in the two groups respectively （P 〈0.05）. （3）Traditional Chinese medicine symptoms： the cure rates were 30.8% vs. 4.0% （PP） and 27.6% vs. 3.6% （ITT） ; and the overall improevment rates were 92.3% vs. 48.0% （PP） and 82.7% vs 42.9% （ITT） in the two groups respectively （P 〈0. 05）. The pain duration after treatment in the TXNG group was significantly shortened compared with the placebo group （7.6 ±4. 6d vs 14. 4 ±4. 3d, P =0. 0125）. After 4-week follow-up, it suggested that the recrudescent duration in symptoms related to IBS in the TXNG group was longer than that in the placebo group （11.5 ±5.3 d vs 6.2 ±6.9 d, P = 0. 019）. No adverse effects were found in the TXNG group. Conclusion It was demonstrated that TXNG is effective and safe in the treatment of diarrhea-predominant ms （stagnation of the liver-qi attacking the spleen）.