ObjectiveTo explore the potential mechanism of Erchen Decoction （二陈汤， ECD） in improving metabolic syndrome （MS） with phlegm syndrome. MethodsForty mice were randomly divided into a blank group of 10 mice and a modeling group of 30 mice. The MS model with phlegm syndrome was induced in the modeling group by high-fat diet. Thirty successfully modeled mice were randomly divided into a model group， a ECD group， and a metformin group， with 10 mice in each group. The ECD group was given 0.4 g/（kg·d） of ECD， while the metformin group was intervened with 11.1 g/（kg·d） of metformin solution， and the blank group and the model group were given 0.02 ml/（g·d） of sterilized drinking water， all by gavage， once daily for 4 weeks. Body weight， abdominal circumfe-rence， body length， Lee's index and food intake were recorded. Blood glucose and blood lipid levels including fasting blood glucose， triglycerides （TG）， total cholesterol （TC）， high-density lipoprotein cholesterol （HDL-C）， and low-density lipoprotein cholesterol （LDL-C） were measured. ELISA was used to detect serum leptin levels， while HE staining was used to observe liver pathological changes. Western Blot and q-PCR were used to detect protein and mRNA expression of hypothalamic leptin receptor （LepR）， pro melanocortin （POMC）， and neuropeptide Y （NPY） in the hypothalamus. Immunofluorescence was used to detect fluorescence expression of POMC and NPY in the hypothalamic arcuate nucleus region. ResultsPathological results showed that the mice in the model group had numerous fat vacuoles in hepatocytes and significant liver fat deposition， while the ECD and metformin groups showed reduced fat vacuoles and less liver fat deposition. Compared to those in the blank group， the mice in the model group mice showed liver fat deposition， increased body weight， abdominal circumference， Lee's index and food intake； fasting blood glucose， TG， TC， LDL-C， and serum leptin levels were elevated， while HDL-C was decreased； the expression of LepR， POMC protein levels and their mRNA expression decreased， while the protein level and mRNA expression of NPY increased； the fluorescence expression of POMC in the arcuate nucleus was reduced， while NPY fluorescence expression increased （P＜0.05 or P＜0.01）. Compared to the model group， the ECD group and metformin group showed significant improvements in the above indicators （P＜0.05 or P＜0.01）. Compared to the ECD group， the metformin group showed a reduction in body weight and NPY fluorescence expression， and an increase in HDL-C levels （P＜0.05 or P＜0.01）. ConclusionECD can downregulate serum leptin levels and improve glucose and lipid metabolism in the MS of phlegm syndrome. Its mechanism of action may be to reduce liver fat deposition and thereafter affect the expression of neuropeptides related to feeding activity in the hypothalamus.