The efficacy of maslinic acid in the treatment of diarrhea-predominant irritable bowel syndrome (IBS-D) was investigated. Rats were divided into five groups: blank control, model, positive control, high-dose maslinic acid, and low-dose maslinic acid. The IBS-D model was set up by acetic acid-constraint-combined stimulation. Fecal water content, abdominal withdrawal reflex, in vitro colon contraction, and pathological staining and 5-hydroxytryptamine (5-HT) immunohistochemistry of colon were detected and analyzed. The in vivo results showed a significant improvement in the fecal water content; and the abdominal withdrawal reflex times returned to the normal range in the high-dose maslinic acid group. The effects were comparable to those of the positive control group. Low-dose maslinic acid exhibited weak activity. No pathological change was observed in any group in vitro. The susceptibility of colon to acetylcholine (Ach) and 5-HT was restored to normal in high-dose maslinic acid group, and 5-HT-positive enterochromaffin cells were reduced. These indicate that maslinic acid had certain efficacy for IBS-D, and high-dose maslinic acid displayed better effects than low-dose.