The natural course of coronavirus disease 2019 (COVID-19) patients without clinical intervention has not yet been documented. One hundred and fifty-eight patients from two hospitals were enrolled to identify the indicators of severe COVID-19 and observe the natural course of COVID-19 patients without clinical intervention. The total computed tomography (CT) score, a quantitative score based on assessment of the number, quadrant, and area of the lesions in CT, tended to perform better than assessment based only on the number or area of the lesions (p = 0.0004 and p = 0.0887, respectively). Multivariate logistic regression showed that the total CT score, chest tightness, lymphocyte, and lactate dehydrogenase (LDH) were independent factors for severe COVID-19. For patients admitted in 2 weeks from onset to hospitalization, the frequency of severe COVID-19 was gradually increased with the delayed hospitalization. The symptoms of fatigue, dry cough, sputum production, chest tightness, and polypnea were gradually more frequent. The levels of C-reactive protein, alanine aminotransferase, aspartate aminotransferase, total bilirubin, direct bilirubin, γ-glutamyl transpeptidase, LDH, and d-dimer were also gradually increased, as well as the scores based on CT. Conversely, the lymphocyte count and the albumin level were gradually decreased with the delayed hospitalization. Detail turning points of the above alterations were observed after 10-14 days from onset to hospitalization. Total CT score was a simple and feasible score for identifying severe COVID-19. COVID-19 patients without clinical intervention deteriorated gradually during the initial 10-14 days but gradually improved thereafter.