The prevalence of cognitive impairment in elderly obese patients is high, and the burden on families and society is heavy. Early intervention for mild cognitive impairment (MCI) is of great value. Central nervous system insulin resistance plays a role in the pathogenesis of cognitive impairment, and functional magnetic resonance imaging(fMRI) can evaluate cognitive impairment by observing central insulin resistance. Some large database studies of Type 2 Diabetes show that metformin is related to reducing the risk of dementia, but some studies have different conclusions, and there is few related study in elderly obese patients. The investigators speculate that metformin may improve cognitive dysfunction by improving central insulin resistance in elderly obese patients. A prospective, randomized controlled single center clinical cohort study will be conducted on 54 elderly obese patients with MCI. One group will receive metformin and lifestyle intervention, while the control group will receive simple lifestyle intervention. All subjects will be followed up for 26 weeks. Medical history collection, physical examination, and laboratory tests will be conducted before and after intervention, and the Montreal Cognitive Assessment Scale will be used for evaluation. Nasal insulin inhalation combined with fMRI will be used to evaluate central insulin resistance status as an objective basis for cognitive function evaluation. The main purpose of the study is to provide more accurate clinical research evidence for the prevention and treatment of MCI in elderly obese patients, in order to reduce the risk of developing dementia and alleviate the burden on families and society.