Objective To observe the changes in pulmonary function in chronic obstructive pulmonary disease (COPD) patients with different GOLD grades, diffusing capacity for carbon monoxide in percent predicted value (DLCO%pred), and the relationship between DLCO%pred and hyperinflation parameter, the ratio of residual volume to total lung capacity (RV/TLC). Methods Their age, sex, body mass index (BMI), and lung function parameters were recorded. Group according to GOLD grade, and compare the lung function among different groups; With DLCO%pred 60% as the cut-off value, the lung function of each group was compared. Pearson and Spearman correlation determined the relationship between DLCO%pred and age, RV/TLC, and other factors. Results Four hundred and thirty-one patients were enrolled. With the increase of GOLD grade, the pulmonary function became worse (P<0.05). Compared with DLCO%pred ≥ 60%, the group with DLCO%pred<60% had lower BMI (t=–5.642, P<0.001) and worse pulmonary function (P<0.001). The correlation analysis showed that BMI (r=0.352, P<0.001), forced vital capacity in percent predicted value (FVC%pred) (r=0.349, P<0.001), forced expiratory volume in one second in percent predicted value (FEV1%pred) (r=0.414, P<0.001), the ratio of forced expiratory volume in one second to forced vital capacity (FEV1/FVC) (r=0.434, P<0.001), peak expiratory flow in percent predicted value (PEF%pred) (r=0.335, P<0.001), maximal mid expiratory flow in percent predicted value (MMEF%pred) (r=0.405, P<0.001), inspiratory capacity in percent predicted value (IC%pred) (r=0.301, P<0.001) were positively correlated with DLCO%pred; while RV/TLC (r=–0.328, P<0.001), GOLD grade (r=–0.430, P<0.001) were negatively correlated with DLCO%pred; However, there was no significant correlation between age (r=–0.012, P=0.810), sex (r=0.076, P=0.117) and DLCO%pred. Conclusions With the increase of GOLD grade, DLCO%pred decreases, and RV/TLC increases. RV/TLC is negatively correlated with DLCO%pred. Attaching importance to the evaluation of hyperinflation and diffusing capacity of COPD patients is helpful in evaluating COPD in all aspects, and has particular clinical significance.