Abstract： Objective:To explore the correlation between the missed early diagnosis time (from the time of clinical symptoms presentation to the time of diagnosis) and the severity of chronic obstructive pulmonary disease (COPD).Methods:This was a case control study.A non-random sampling method was used to select 160 patients for the first diagnosis of COPD who were admitted to the Liu′an Hospital affiliated to Anhui Medical University from February 2020 to June 2022.Patients were divided into COPD ( n=120) and asthma-COPD overlap (ACO) ( n=40) according to the global guidelines for the prevention and treatment of COPD.General clinical data of the patients were collected.The relationship between patients′ age, gender, disease classification, missed early diagnosis time, and disease severity was analyzed.Multivariate linear regression analysis was used to analyze multiple factors. Results:There was no significant difference in missed early diagnosis time and forced expiratory volume in the first second in percent predicted values (FEV 1%pred) in patients of different genders and of different disease types (all P>0.05).There were significant differences in missed early diagnosis time and FEV 1%pred in different age groups (all P<0.05).The missed early diagnosis time of 60-69 years old and ≥70 years old group was lower than that of <50 years old and 50-59 years old, and the FEV 1%pred was higher than that of <50 years old and 50-59 years old, the difference was statistically significant (all P<0.05).The difference of FEV 1%pred in different missed early diagnosis periods was statistically significant ( F=2.74, P<0.05).The FEV 1%pred of 1≤ missed early diagnosis time <5 years group, 5≤ missed early diagnosis time <10 years group and missed early diagnosis time ≥10 years group were lower than those of missed early diagnosis time <0.5 years group and 0.5≤ missed early diagnosis time <1 year group.5≤ missed early diagnosis time <10 years group and ≥10 years group and less than 1≤ missed early diagnosis time <5 years group, the difference was statistically significant (all P<0.05).Multiple linear regression showed that age and missed early diagnosis time were the factors influencing the severity of COPD. Conclusions:Missed early diagnosis time is the influencing factor of COPD severity.It is necessary to identify the disease early in the clinical practice to reduce the incidence of missed diagnosis.