Since the outbreak of COVID-19, 2-3% of COVID-19 patients died in short time. Immune responses may play critical role in this process. The numbers of T cells and NK cells were reduced in SARS-CoV-2-infected patients, which was probably due to the combinational interactions of these cells. Our study showed a decreased T cell proportion with down-regulated gene expression related to T cell activation and differentiation were occurred in COVID-19 severe patients, which may help to understand the immune response and provide effective treatment strategies for COVID-19.