Abstract: Objective To explore the application value of CD4⁺ T cell count and CD4⁺ T/CD8⁺ T ratio in the early diagnosis and treatment evaluation of patients with coronavirus disease 2019 (COVID-19). Methods From January 20, 2020 to March 4, 2020, 101 patients diagnosed with COVID-19 in Fuyang Second People's Hospital were selected as the research subjects. The differences of first time CD4⁺ T cell count and CD4⁺ T/CD8⁺ T cell ratio in peripheral blood between common patients (n=81) and severe-critical patients (n=20) were analyzed retrospectively, and the effects of gender, age, disease severity and underlying medical conditions on T cell count were analyzed. Results The first time count of CD4⁺ T cells in the severe-critical patients was significantly lower than that of the common patients (315.35±50.56)/μl vs. (467.00±21.38) /μl), the difference was statistically significant (P<0.05). There was no significant difference in CD4⁺ T/CD8⁺ T cell ratio between the two groups (P>0.05). The factors influencing CD4⁺ T cell count were age, CD4⁺ T/CD8⁺ T cell ratio and clinical classification of COVID-19. Conclusions The immune function of patients with COVID-19 is affected. The decrease of CD4⁺ T cell count in severe-critical patients is more serious than that in common patients. Attention should be paid to the immune function of patients aged older than 50 years and those with underlying medical conditions.