Abstract Background Coronavirus disease 2019 (COVID-19) is a novel infectious viral disease, which lacks well-established diagnostic laboratory parameters that could be used to evaluate disease severity, thromboembolism or cardiovascular events and to predict clinical prognosis. Coagulation cascade and platelet functions have not been well studied in the COVID-19 patients. Methods A total of 178 patients enrolled in Wuhan Huoshenshan Hospital were included for the study. Blood platelets and coagulation functions were analyzed in COVID-19 patients with non-severe and severe subgroups. Other biochemical laboratory parameters were also analyzed. Results Forty-nine (27.5%) out of 178 patients were diagnosed with severe disease in this study, and 129 patients with non-severe disease. Severe disease group had significant lower platelet count 186.00 (103.50–249.00) ×10⁹/L than 251.00 (202.00–317.00) ×10⁹/L of non-severe group, p = 0.000. Severe group also had significantly abnormal coagulation parameters than non-severe group: prothrombin time (PT) 14.55 (13.40–16.53) s vs. 12.70 (12.15–13.59) s, p = 0.000; international normalized ratio (INR) 1.21 (1.13–1.36) vs. 1.06 (1.01–1.13), p = 0.000; thrombin time (TT) 16.35 (15.69–17.47) s vs. 15.68 (14.79–16.69) s, p = 0.011; D-Dimer 1.05 (0.68–5.90) mg/L vs. 0.42 (0.28–0.79) mg/L, p = 0.000; While the liver function parameter alanine aminotransferase (ALT) and aspartate aminotransferase (AST) didn’t show significance between two groups, ALT 30.80 (19.00–58.30) IU/L vs. 28.80 (15.75–50.15) IU/L, p = 0.487; AST 27.80 (19.30–40.55) IU/L vs. 22.6 (16.7–32.03) IU/L, p = 0.102. Disseminated intravascular coagulation (DIC) rate was 6.1% in severe group while 0% in non-severe group. Survival rate of severe disease group was worse than non-severe group, 85.7% vs. 100%, p = 0.000. Thrombocytopenia correlated with coagulation function, DIC rate and survival. Six out of 7 death case had thrombocytopenia during hospitalization, and platelet count decreased subsequently until death. Thrombocytopenia occurred within 1 week after admission in 6 recovered patients. And increased platelet levels followed by positive SARS-CoV-2 IgM/IgG and negative coronavirus nucleic acid tested in 8 recovered patients. Conclusions Low platelet count is associated with abnormal coagulation function and increased risk of DIC, severe disease manifestation and increased mortality in patients with COVID-19.