Objective To investigate effects of Sinisanhetong decoction on 5-HT in patients with irritable bowel syndrome(IBS).Methods Sixty cases of diarrhea-predominant IBS patients were randomly divided into two groups,30cases in each group,all cases in the control group were treated with Trimebutine+triple viable,and those in the observation group treated with Sinisanhetong decoction,the main symptom scores and serum 5-HT level changes were compared between the two groups before and after treatment.Results After one course of treatment,the abdominal pain,diarrhea,bloating,stool urgency,anal and other major symptom scores and serum concentration of 5-HT in the two groups were significantly lower than those before treatment,and those in the the observation group were lower than those in the control group,the differences were statistically significant(P0.05);The total effective rate in the observation group was 83.33%,and it was significantly higher than the 60.00%in the the control group, the difference was statistically significant(P0.05).Conclusion Sinisanhetong decoction can regulate the normal regulation of 5-HT secretion in patients with IBS,and it can improve the gastrointestinal motility disorders and intestinal secretion abnormalities.