Objective To analyze the clinical and epidemiological characteristics of coronavirus disease 2019 (COVID-19) patients who were complicated with diabetes mellitus (DM), so as to determine whether DM is a risk factor for disease progression and poor prognosis of COVID-19. Methods A total of 856 hospitalized patients who were confirmed with COVID-19 in Zhejiang Province from January 17 to February 7, 2020 were collected, including 64 patients with DM. Clinical characteristics, epidemiology, treatment and prognosis of patients with and without DM were compared and analyzed. Results Age of 64 patients with DM were (58.1±11.2) years, which was higher than that of patients without DM (P < 0.001), the most common symptoms at onset included fever (89.1%), cough (71.9%), expectoration (39.1%) and fatigue (25.0%), patients with DM were more likely to develop muscle soreness (P=0.047). Compared with patients without DM, serum inflammatory-related markers (such as neutrophils and C-reactive protein) in patients with DM increased (P < 0.05), proportion of severe cases was higher, and acute respiratory distress syndrome was more prone to occur (P < 0.01). After adjusting smoking and gender factors, the risk of reaching the composite endpoint (including admission in ICU due to disease progression, shock, mechanical ventilation and death) in DM patients was higher than that in non-diabetic group, and the hazard ratio (HR) was 3.50 (95%CI: 1.51-8.11). Conclusion COVID-19 patients with DM have a higher age and higher proportion of severe cases and are more likely to cause serious adverse outcomes, DM should be regarded as a risk factor for disease progression and poor prognosis of COVID-19.