Introduction Diabetes is one of the most common comorbidities of COVID-19. We aimed to conduct a multidimensional analysis of risk factors associated with the severity and mortality of patients with COVID-19 and diabetes. Methods In this retrospective study involving 1443 patients with COVID-19, we analyzed the clinical and laboratory characteristics and risk factors associated with disease severity in patients with COVID-19 with and without diabetes. Binary logistic regression analyses were performed to identify the risk factors associated with mortality in patients with COVID-19 and diabetes. The 84-day survival duration for critical patients with COVID-19 and diabetes who had different levels of leukocytes and neutrophils, or treated with immunoglobulin or not, was conducted using Kaplan-Meier survival curves. Results Of the 1443 patients with COVID-19, 256 (17.7%) had diabetes, had a median age of 66.0 [IQR 58.0-73.8] years, and were more likely to develop severe (41.8% vs. 35.6%) and critical disease (34.0% vs. 14.9%), followed by higher mortality (21.1% vs. 7.0%), than those without diabetes. Higher levels of leukocytes (> 5.37 x 10(9)/L), older age, and comorbid cerebrovascular disease and chronic renal disease independently contributed to in-hospital death of patients with COVID-19 and diabetes. Leukocytes > 5.37 x 10(9)/L and the application of immunoglobulin were associated with shorter survival duration and lower mortality, respectively, in critical patients with COVID-19 and diabetes. Conclusions More attention should be paid to patients with COVID-19 and diabetes, especially when they have high leukocyte counts (> 5.37 x 10(9)/L). Timely and adequate intravenous immunoglobulin (IVIG) use may reduce the mortality of critical patients with COVID-19 and diabetes.