Abstract: Huang-Kui-Si-Wu Formula (HKSWF) can reduce the accumulation of uremic toxin p-cresyl sulfate (PCS) and its precursor p-cresol (PC) in a rat model of chronic kidney disease (CKD) and delay the progression of CKD. However, the mechanism by which HKSWF decreases PC accumulation is not clear. This study investigated the effect of HKSWF on PC production in intestinal microbes as well as its mechanism of action. After CKD model rats were given HKSWF by intragastric administration, feces were collected to analyze the gut bacterial composition by 16S rDNA sequencing technology. All procedures were approved by the Institutional Animal Care and Use Committee of the Nanjing University of Chinese Medicine. The results showed that HKSWF inhibited PC production without decreasing the abundance of harmful bacteria. HPLC-UV-FLD was used to detect p-cresol. An in vitro anaerobic culture system was used to study the effect and mechanism of action of HKSWF on PC production in gut bacteria. The results show that food-derived tyrosine (Tyr) could significantly promote PC production in intestinal bacteria, and HKSWF (4000, 400, 40 μg·mL^-1) could dose-dependently inhibit PC production in gut bacteria in vitro. HKSWF inhibited bacterial PC synthesis by two pathways: it decreased the oxidation pathway from 82.83% to 38.87%, and increased the reductive pathway from 17.17% to 61.13%. This result suggests that HKSWF could inhibit PC production by switching tyrosine metabolism from an oxidative pathway to a reductive pathway. Secondly, HKSWF could directly inhibit the oxidative pathway of tyrosine and decrease the decomposition of PHA, thereby inhibiting PC production. These results suggest that HKSWF could inhibit the formation of harmful uremic toxins by modulating the metabolic pathway of PC in gut microbiota and thereby delaying CKD progression.