Objective To study the effect of walnut oil on lipid metabolism and elucidate its mechanism of action.Methods A high-fat diet is used to establish a high-fat model of SD rats, and 500, 1 000, and 1 500 mg/kg undiluted walnut oil is used for gavage intervention experiments. After 8 weeks of intervention, the changes in body weight, organ indexes, serum lipid biochemical indicators, and serum antioxidant related indicators of SD rats are measured to observe the effects of walnut oil on organs, blood lipids, and antioxidant capacity. The expression of ABCA1 mRNA, ABCG1 mRNA, and SREBP2-HMGCR signaling pathway and PPARα-LXRα-CYP7A1 signaling pathway genes in rat liver are measured by using the QPCR method to explore the mechanism by which walnut oil regulates liver cholesterol metabolism.Results Compared with the high-fat model group, walnut oil can significantly reduce the body weight and liver index of SD rats (P<0.05), but has no effect on the heart index, kidney index, and spleen index. Walnut oil significantly reduces TC, TG, and LDL in the serum of SD rats (P<0.05). It can also significantly increase HL and LPL content to regulate LDL and HDL (P<0.05). In addition, walnut oil significantly increases the content of SOD, GSH-Px, CAT, and T-AOC (P<0.05) and significantly reduces the content of MDA (P<0.05). Walnut oil significantly upregulates the expression of ABCA1 mRNA and ABCG1 mRNA (P<0.05), downregulates the expression of SREBP2 mRNA and HMGCR mRNA, and upregulates the expression of PPARα mRNA, LXRα mRNA and CYP7A1 mRNA.Conclusion Walnut oil can reduce serum TC levels by reducing liver TC biosynthesis and enhancing liver TC cell uptake, decomposition, and excretion and also improves the imbalance of the antioxidant system by increasing the expression of antioxidant system enzymes.