Background: The stage of CT images was rarely studied and the relationship between the severity of Coronavirus Disease 2019 (COVID-19) and CT images has not been studied based on systematic quantitative analysis currently. Purpose: To investigate the staging duration and classification of CT images of patients with COVID-19 based on quantitative analysis. Materials and Methods: This is an ambispective observational cohort study based on 125 patients with COVID-19 from Jan 23 to Feb 28, 2020. The stage of CT and pulmonary lesion size were quantitatively analyzed. The categorical regression analysis based on optimal scale (CATREG) was performed to evaluate the association of CT score, age, and gender with the clinical type. Results: The CT images of 125 patients with COVID-19 (50.13 plus-or-minus 16.91 years, 66 women) were analyzed in this study. Except for pre-early stage, the duration of early, progression-consolidation, and dissipation stage of CT images was 3.40 plus-or-minus 2.31, 10.07 plus-or-minus 4.91, and 20.60 plus-or-minus 7.64 days, respectively. The median CT score was 5.00 (2.00-8.50) during the first 30 days, which reached a peak on the 11th day. Significant differences were found between the median CT scores of different clinical types (P less than 0.05). Besides, the age was correlated with the clinical type (P less than 0.001), the CT scores of 0.00-11.50, 11.50-16.00, and 16.00-20.00 were separately correlated with the moderate, severe, and critical type with the output accuracy 69.60%. Conclusion: The four-stage staging method based on quantitative analysis is consistent with the change rules of staging features and COVID-19. Quantitative study by scoring pulmonary lesion sizes accurately revealed the evolvement of pulmonary lesions and differences between different clinical types.