Objective: To explore the long-term effects of SARS-Cov-2 infection on the pulmonary function in convalescent COVID-19 patients of 6 to 9 months follow-up in Beijing, China. Methods: 64 cases of COVID-19 patients were recruited for the study, discharged from the Beijing Ditan Hospital, Capital Medical University for 6 to 9 months. COVID-19 patients were divided into mild, moderate and severe groups. The pulmonary function tests, the novel coronavirus antibody (IgM and IgG), chest CT and blood tests were investigated during follow-up. Results: 31.2% (20/64) patients had pulmonary ventilation dysfunction and 35.9% (23/64) had diffusion dysfunction. In the severe group, 56.50% (13/23) individuals showed decreased diffusion function. The diffusion dysfunction of severe group was significant decreased than the moderate (P=0.021). Among 56 cases, the positive rate of IgG titers was 73.2% (41/56). The result of chest CT showed 55.4% (31/56) cases in nodules, 44.6% (25/56) in strip-like changes, 37.5% (21/56) in ground glass shadow. Patients were tended to have ground glass changes in the severe group, while nodules in the moderate group. Conclusion: For the 6 to 9 months in convalescent COVID-19 patients, 56.50% (13/23) severe patients had pulmonary diffusion dysfunction. In the convalescent COVID-19 patients, especially those with severe illness, should have their pulmonary function tested regularly.