Background Major depressive disorder (MDD) is a chronic common mental disorder. Long-term medication may lead to abnormalities in vivo factors, which cause obesity.Aim To find out the related factors of obesity by analyzing the metabolic indexes of patients with major depressive disorders of all ages in stable stage.Methods Subjects with major depressive disorder (MDD) were treated with fixed dose drugs and conventional drugs for 2 years or more. Venous blood was collected and blood metabolic indexes were analyzed.Results Among MDD, with the increase of age, the risk of obesity increases. Body mass index (BMI) was positively correlated with age, total cholesterol (TC), triglyceride (TG), low density lipoprotein (LDL), aspartate aminotransferase (AST), lactate dehydrogenase (LDH), uric acid (UA), alanine aminotransferase (ALT), glycosylated hemoglobin (HbA1c) and high sensitivity C-reactive protein (HsCRP), and high density lipoprotein (HDL). But creative kinase isoenzyme (CK-MB), urea, serum creatinine (SCr) and homocysteine (Hcy) were not correlated with BMI. Multiple linear regression analysis showed that the value of BMI increased with the increase of age (B = 0.050, p = 0.002), AST (B = 0.094, p = 0.049), ALT (B = 0.055, p = 0.016), UA (B = 0.006, p < 0.021), HbA1c (B = 0.702, p = 0.004) and hsCRP (B = 0.101, p < 0.001). Discussion We should pay more attention to the monitoring of blood metabolism indicators with the increase of age in MDD, and timely adjust the use of drugs timely adjust the use of drugs refer to their liver and kidney function, so as to reduce the risk of obesity and improve their quality of life.