Abstract Background The effects of a delayed diagnosis of asthma on lung function in children have not been well investigated. Therefore, a retrospective cohort study was conducted in a children’s hospital to analyse the effect of delayed diagnosis time on lung function in children with asthma. Methods We conducted a retrospective cohort study in Jinan Children's Hospital from January 1, 2010, to December 31, 2020. All children were divided into different groups according to the presence or absence of rhinitis, age at first onset (first coughing and wheezing attack) and delayed diagnosis duration (≤ 3 months, 3–12 months, 1–3 years, 3–5 years and > 5 years). Results A total of 1,014 children with asthma were included in this study. The median (quartile) delay in asthma diagnosis among all participants was 11 (2, 26) months. The shortest delay in diagnosis time was on the same day of onset, and the longest delay in diagnosis time was 10 years. The median (quartile) duration of delayed diagnosis was 10 (2, 26) months in 307 asthmatic children without rhinitis and 11 (2, 26) months in 707 children with asthma and rhinitis (P 3 years (P 0.05) on FVC%pred and FEV1%pred. Conclusion Although delayed asthma diagnosis can lead to lung function impairment in children with asthma, lung function can be improved quickly after standardised treatment. Therefore, early asthma diagnosis and standardised treatment are very important.