To investigate the therapeutic effects of Zengye Chengqi Decoction on chronic transit - type constipation (STC) in rats and its underlying molecular mechanisms, particular attention was paid to the regulation of water channel protein expression, specifically AQP3 and AQP9. An STC rat model was established via intragastric administration of Compound Diphenoxylate Tablets. Eighty - four SD rats were randomly assigned to seven groups: normal control, model control, low - dose Zengye Chengqi Decoction (9.72 g/kg/d), medium - dose Zengye Chengqi Decoction (19.44 g/kg/d), high - dose Zengye Chengqi Decoction (38.88 g/kg/d), positive drug control (Itopride 13.5 mg/kg/d), and a combination group receiving high - dose Zengye Chengqi Decoction plus U0126 (0.1 mg/kg/d). The intervention lasted for 14 days. Results demonstrated that the medium - and high - dose Zengli Chengqi Decoction groups, as well as the combination group, significantly increased fecal output and water content (P < 0.05), shortened the time to first black stool excretion, enhanced intestinal propulsion rates, and improved colonic myoelectric activity (increased frequency and reduced amplitude). Molecular mechanism studies revealed that Zengye Chengqi Decoction inhibits the MAPK/ERK signaling pathway, downregulates AQP3 expression, and upregulates AQP9 expression (P < 0.05). Notably, the efficacy of the high - dose Zengye Chengqi Decoction group was comparable to that of the positive drug group (P > 0.05). In conclusion, Zengye Chengqi Decoction may improve intestinal transmission function in STC rats by modulating the MAPK/ERK - AQP3/AQP9 pathway, providing a potential experimental basis for clinical applications.