Background Novel severe acute respiratory syndrome coronavirus 2 causes the novel coronavirus disease (COVID-19) in humans, which has spread rapidly worldwide. Most critical cases of COVID-19 are accompanied by complicated chronic underlying diseases. This retrospective study aims to analyze the clinical characteristics of COVID-19 patients with chronic underlying diseases.Methods A total of 1,183 COVID-19 patients were divided into the chronic underlying disease (CUD, n = 538) group and the non-underlying disease (non-CUD, n = 645) group. The clinical characteristics and outcomes were collected and compared between the two groups.Results There were significant differences in age, weight, and SPO2 on admission between the CUD and non-CUP groups. The ratio of severe cases in the CUD group was higher than that in the non-CUD group (χ 2 = 35.58, p -value < 0.001). The white blood cell count, neutrophil count, C-reactive protein, urea nitrogen, creatinine, myoglobin, and cardiac troponin in the CUD group were significantly higher than those in the non-CUD group, while the lymphocyte count and albumin in the underlying disease group were significantly lower than those in the non-underlying disease group. No significant difference was found in the total number of tests, the number of positive or negative results in nucleic acid tests between the two groups. The negative rate for both IgG- and IgM-antibody tests in the CUD group was higher than that in the non-CUD group (χ 2 = 5.57, p -value = 0.018). No statistical difference in mortality between the CUD (n = 18) and non-CUD groups (n = 13). All surviving patients were cured and discharged. A total of 33 patients had a positive re-examination result for nucleic acid test one week after discharge, including 14 patients with underlying diseases and 19 patients without underlying diseases. Conclusion : COVID-19 patients with underlying diseases had poorer clinical conditions and had a longer hospital stay, but after active treatment, the mortality had not increased significantly.