Abstract: The purpose of this study was to observe the fat reducing effect of lemon juice fermented by Lactobacillus plantarum KFY02 and its regulation on intestinal flora. This study induced obesity in mice through the establishment of high-fat diet, the experimental mice were divided into normal group, model group, Lactobacillus plantarum KFY02 group, unfermented lemon juice group and fermented lemon juice group, and biochemical kits, section observation and quantitative PCR were used to detect the related indexes of mouse serum and tissue, and the composition of intestinal microorganisms was observed by detecting the expression of microorganisms in feces. The results showed that Lactobacillus plantarum KFY02, unfermented lemon juice and fermented lemon juice could effectively reduce the body weight of obese mice, reduce the organ index of liver and epididymal adipose tissue in obese mice, and reduce the levels of alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (AKP), total cholesterol (TC), triglyceride (TG), low density lipoprotein cholesterol (LDL-C) and raise high density lipoprotein cholesterol (HDL-C) in serum and liver tissue of obese mice, and also could increase the contents of total cholesterol (TC), triglyceride (TG) and total protein (TP) in mouse feces. H&E pathological observation showed that Lactobacillus plantarum KFY02, unfermented lemon juice and fermented lemon juice could also reduce liver tissue lesions and adipocyte enlargement and accumulation caused by obesity. qPCR results showed that Lactobacillus plantarum KFY02, unfermented lemon juice and fermented lemon juice could up-regulate PPAR-α, CPT-1, LPL, CYP7A1 and down-regulate C/EBPα, PPAR-γ mRNA expression in liver tissue of obese mice induced by high fat diet, meanwhile, they could also up-regulate the expression of Bacteroidetes, Lactobacillus sp., Bifidobacterium sp. and down-regulate the expression of Firmicutes. These results showed that Lactobacillus plantarum KFY02, unfermented lemon juice and fermented lemon juice could reduce fat in mice, and the effect of fermented lemon juice was the best. The fat reduction effect of fermented lemon juice on mice was directly related to its regulation of intestinal function and microbial composition. It could be seen that lemon juice fermented by Lactobacillus plantarum KFY02 had good lipid-lowering and intestinal regulation in obese mice.