Objective To analyze the effect of body mass index (BMI) on pulmonary function and quality of life in elderly patients with chronic obstructive pulmonary disease (COPD), and to provide theoretical basis for the diagnosis and treatment of COPD. Methods A total of 1 278 patients who visited the respiratory department of our hospital from January 2019 to June 2022 were selected as the research objects, including 368 COPD patients, 225 of whom were in stable stage and 143 of whom were in acute exacerbation stage. According to BMI, they were divided into low group (n=31, BMI<18.5kg/m²), normal group (n=198,18.5kg/m2≤BMI<24 kg/m²), overweight group (n=106, 24kg/m²≤BMI<28 kg/m²) and obese group (n=33,BMI≥28 kg/m²). Health-related quality of life (HRQOL) was used to evaluate the quality of life of patients. Forced expiratory volume in one second (FEV1), forced vital capacity (FVC) and FEV1/FVC were measured by pulmonary function instrument to evaluate the pulmonary function grade of patients. Logistic regression was used to analyze the risk factors of pulmonary function and quality of life in COPD patients. Results There were significant differences in age, smoking, BMI and hyperlipidemia between stable COPD patients and acute exacerbation COPD patients with different BMI groups (P<0.05). There were significant differences in the proportion of pulmonary function Ⅲ/Ⅳ, FEV1/FVC, FEV1pred and HRQOL scores among different BMI groups (P<0.05) . The FEV1/FVC and FEV1pred in patients with acute exacerbation and stable COPD from high to low were obese group, overrecombination group, normal group and too low group, and the differences were statistically significant (P<0.05). The HRQOL scores of COPD patients in acute exacerbation and stable stage from high to low were obese group, overweight group, normal group and too low group, and the difference was statistically significant (P<0.05). Low body mass index was a risk factor for pulmonary function in COPD patients at stable stage (OR=2.52) and acute exacerbation stage (OR=2.83) (P<0.05). High body mass index was a risk factor for quality of life in patients with stable COPD (OR=2.43) and acute exacerbation (OR=2.65) (P<0.05). Conclusion BMI can affect pulmonary function and quality of life in COPD patients. Patients should maintain normal BMI, which can improve pulmonary function and quality of life.