Chansu and its major active constituent of bufalin have been reported to have broad-spectrum antiviral effects. This study aims to assess the efficacy of Chansu injection in treating patients with severe COVID-19. The patients diagnosed as severe or critical COVID-19 in The First People's Hospital of Jiangxia District, Wuhan, China from February 5 to March 5, 2020 were randomly allocated in a 1:1 ratio to receive general treatment plus Chansu injection or only general treatment as the control group. The treatment course was 7 days. The changes of PaO2/FiO2 and ROX index indicating respiratory function, the white blood cell (WBC) count, peripheral blood mononuclear lymphocyte (PBML) count, respiratory support step-down time (RSST), and safety indicators for the 7th day were retrospectively analyzed. After 7 days treatment, the oxygenation index was improved in 20 of 21 patients (95.2%) in the treatment group, as compared with 13 of 19 patients (68.4%) in the control group. The PaO2/FiO2 and ROX indices in the treatment group (mean, 226.27 and 14.01 respectively) were significantly higher than the control group (mean, 143.23 and 9.64 respectively). The RSST was 1 day shorter in the treatment group than the control group. Multivariate regression analysis suggested that Chansu injection contributed the most to the outcome of PaO2/FiO2. No obvious adverse effects were observed. Preliminary data showed that Chansu injection had apparent efficacy in treating patients with severe COVID-19.