Abstract: Objective To investigate the association of the time of initial diagnosis with the severity of chronic obstructive pulmonary disease (COPD). Methods A total of 803 patients who were diagnosed to have COPD for the first time in our hospital between May 2015 to February 2018 were enrolled in this study. The diagnoses of COPD and asthma COPD overlap (ACO) were made according GOLD guidelines and european consensus definition. Lung function of the patients was graded according to the GOLD guidelines. Results The patients with COPD had a mean age of 61.8±9.9 years, including 726 male and 77 female patients. The course of the patients (defined as the time from symptom onset to the establishment of a diagnosis) was 3（0.5, 8）years. Among these patients, 85.2% had a moderate disease severity (FEV1%<80%), and 48.3% had severe or very severe conditions (FEV1%<50%); 47.0% of them were positive for bronchial dilation test. In the overall patients, 295 (36.7%) were also diagnosed to have ACO, and the mean disease course of ACO [3 (1, 9) years] was similar to that of COPD [3 (0.5, 8) years] (P>0.05). A significant correlation was found between the disease course and the lung function of the patients. Multiple linear regression analysis showed that an older age and a longer disease course were associated with poorer lung functions and a greater disease severity. Conclusion The delay of the initial diagnosis is significantly related to the severity of COPD.