Coronavirus disease-2019 (COVID-19) is a rapidly escalating epidemic caused by SARS-CoV-2. Identification of a simple and effective indicator to assess disease severity and outcome is urgently needed.  This study retrospectively analyzed dynamic changes of lymphocyte percentage (LYM%) in 15 death cases, 15 severe cases as well as 40 moderate cases of COVID-19 patients. Next, prognostic role of lymphopenia in COVID-19 were verified in 92 hospitalized cases. Our results from death and severe cases showed that LYM% in blood test were inversely associated with the progression and severity of COVID-19. LYM% in patients with moderate COVID-19 remained higher than 20% 10-12 days after symptom onset. In contrast, LYM% was lower than 20% in severe cases. However, LYM% in severe cases was higher than 5% 17-19 days after the onset of the disease, while it fell below 5% in death cases. Therefore, we established a reliable Time from symptom onset-LYM% model (TLM), which could be used to evaluate disease severity and predict the outcomes of hospitalized patients with COVID-19. Authors therefore conclude that Lymphopenia can be used to indicate clinical course, treatment effect and outcomes of COVID-19 patients.