Oxidative stress is present in chronic obstructive pulmonary disease (COPD); however, the effect of increased dietary antioxidants on reducing COPD risk remains unclear. The aim of this study was to investigate the association of the Composite Dietary Antioxidant Index (CDAI) with COPD in adults. This study conducted a cross-sectional investigation using data from the National Health and Nutrition Examination Survey (NHANES) spanning from 2015 to March 2020 to explore the association between CDAI and COPD in adults. This study included 9295 participants. Three logistic regression models (crude model, partially adjusted model, and fully adjusted model) and restricted cubic spline (RCS) curves were utilized to assess the association between CDAI levels and COPD risk. Subsequently, a two-sample Mendelian Randomization (MR) was employed to analyze the causal impact of antioxidant levels within CDAI on the occurrence of COPD. CDAI levels were inversely associated with COPD after adjusting for confounders (OR=0.97, 95%CI:0.95-1.00), and the association was linear (p<0.001), and the results of the RCS showed that CDAI was linearly correlated with COPD occurrence (p<0.001). MR analysis revealed a causal relationship between vitamin C and COPD occurrence (OR=0.99, 95%CI:0.98-1.00, p<0.05). Our study indicates that dietary sources of antioxidants may reduce the risk of COPD occurrence, and the results of the MR analysis further show that vitamin C is causally associated with a reduced risk of COPD occurrence. However, further exploration is needed to understand how antioxidants prevent COPD.