This study aimed to determine the optimal threshold for quantifying air trapping using the subtraction method on co-registered CT in COPD patients and correlate it with pulmonary function parameters. A total of 174 patients were included, and inspiration and expiration CT scans were performed, followed by non-rigid registration. The subtraction value per voxel between the scans was obtained, and the volume fraction of air trapping (air trapping index, ATI) was calculated using various thresholds. ATI, along with other parameters, was correlated with FEF25–75% and RV/TLC. The results showed that the highest correlation coefficients with FEF25–75% and RV/TLC were achieved at a threshold of 60 HU. These correlation coefficients were comparable to those obtained with other methods such as E/I MLD and Exp−856. The findings suggest that a threshold of 60 HU is optimal for quantifying air trapping and shows similar correlations with pulmonary function parameters. The idea of this research has been transferred to AVIEW COPD by Coreline Soft.