Bariatric surgery is the most effective treatment for obesity, type 2 diabetes and related diseases. the mechanism of bariatric surgery improve metabolism have not been fully elucidated. The changes of gastrointestinal anatomy caused by bariatric surgery are the basis of postoperative metabolic improvement, it is generally believed that bariatric surgery reduces patients′ weight and improve metabolism by restricting food intake and reducing food absorption. In addition, increased secretion of the gastrointestinal hormones glucagon⁃like peptide⁃1 and peptide YY and decreased secretion of ghrelin can inhibit gastrointestinal peristalsis, decrease secretion of digestive juice, and decreased food intake. Postoperative bile acid increase and hepatointestinal circulation decrease. The decrease of firmicutes and the increase of proteobacteria and bacteroidetes in postoperative intestinal flora can cause the decrease of fat reserve and the change of fatty acid structure. The decreased secretion of adipokines leptin resulted in decreased food intake, while the increased secretion of adiponectin improved insulin sensitivity. The changes of central nervous system after operation can reduce the reactivity of food reward and reduce the intake of food. All these changes can improve postoperative metabolic status. Besides, the reason for metabolic improvement after surgery is not a single factor, but a result of mutual influence and common changes. This paper reviews the research progress on the mechanism of metabolic improvement after bariatric surgery.