This study is a randomized double-blind placebo-controlled clinical trail to evaluate the clinical efficacy and safety of 1.1 chemical drugs N-acetyl-D-glucosamine on Chinese IBS-D patients coming from thirty- six centers in Chinese. 720 IBS-D patients (360 for treatment group, 360 for placebo group) in this research are accord with the Rome III diagnostic criteria, screening/import period pain intensity scores of the NRS(numerical rating scale) week mean value are 3.0 plus and the days which at least more than one time a stool type are 6 or 7 type over 2 days/week. Test cycle includes screening/import period (2 weeks), open treatment period (12 weeks), follow-up period (2 weeks). The main outcome measures are pain intensity (NRS score 11 point scale) and stool type ( Bristol type), and secondary endpoints included overall symptoms sensory scores, defecation frequency, abdominal distension, defecation urgency and quality of life parameters (IBS-QOL scale).