Aims/Introduction This study aimed to explore the association between glycemic control before admission with severity and mortality of coronavirus disease 2019, and tried to reveal the mechanism. Materials and Methods A total of 77 inpatients were grouped into sufficient control group (glycated hemoglobin [HbA1c] <6.5%, n = 49) and insufficient control group (HbA1c >= 6.5%, n = 28). Regression models were used to analyze the clinical data. Results Compared with patients with HbA1c <6.5, patients with HbA1c >= 6.5 showed higher heart rate (101 vs 89 b.p.m., P = 0.012), lower percutaneous oxygen saturation (93 vs 97%, P = 0.001), higher levels of multiple indicators of inflammation, such as white blood cell count (7.9 vs 5.9 x 10(9)/L, P = 0.019), neutrophil count (6.5 vs 4.1 x 10(9)/L, P = 0.001), high-sensitivity C-reactive protein (52 vs 30 mg/L, P = 0.025) and serum ferritin (1,287 vs 716 mu g/L, P = 0.023), as well as lower levels of lymphocyte count (0.7 vs 0.8 x 10(9)/L, P = 0.049) at hospital admission. Thus, patients with HbA1c >= 6.5 were more likely to develop secondary respiratory infections (25 [89%] vs 33 [67%], P = 0.032) and acute respiratory distress syndrome (17 [61%] vs 14 [29%], P = 0.006) than patients with HbA1c <6.5, resulting in a higher proportion of critically ill patients (19 [68%] vs 18 [37%], P = 0.009) and non-survivors (13 [46%] vs 11 [22%], P = 0.029). After adjustment for potential risk factors, HbA1c was independently associated with in-hospital death. Conclusion HbA1c was an independent risk factor for poor outcomes in coronavirus disease 2019 patients. Severe pulmonary infection and consequent acute respiratory distress syndrome might be the primary causes of death in insufficient glycemic control patients.