Objective To study the effect and the mechanism of Tongxieyaofang (TXYF) on rat model of irritable bowel syndrome (IBS). Methods Model rats of IBS which were prepared by chronic stimulation in colon were randomly divided into groups: the normal control group (Group A), the model control group (Group B), the positive control group (Group C), the low dosage of TXYF group (Group D), the high dosage of TXYF group (Group E). Different treatments were given to every groups by ig administration for one month. The capability limens of the sacculus that caused abdomen-uplifting and back-arching and the times of contract of abdomen muscle in rats under different dilatations were observed. The levels of 5-hydroxytryptamine (5-HT), substance P (SP), and calcitonin gene related peptide (CGRP) were evaluated. Results Compared with the Group B, the TXYF could effectively improve the index of the behavior and electrophysiology, it also could lower the levels of 5-HT and SP, and increase the levels of CGRP of rat model of IBS with certain dose-effect relationship. Conclusion TXYF could lower the levels of 5-HT in serum and SP in plasma, and increase CGRP level of model rats of IBS, especially in high dosage. The mechanism of TXYF may be increasing the pain threshold of bowel, eliminating hypersusceptibility of bowel, and decreasing excitability of nerve cells by decreasing the 5-HT and SP levels in IBS rats.