Objective: To investigate the effect of corticosteroids therapy on the inflammatory response in a critically ill coronavirus disease 2019 (COVID-19) patient. Methods: A 55-year old female patient with critical ill COVID-19 was admitted in Taizhou Hospital on January 19, 2020. The patient was treated with methylprednisolone 80 mg on the 2nd day after admission. Thereafter, the dose was adjusted in a timely manner and the therapy lasted for 13 days. The peripheral lymphocyte subsets (CD3+T, CD4+ T, CD8+ T, NK cells, B cells), as well as serum levels of lymphocyte factors (IL-2, IL-4, IL-6, IL-10, TNF-α, IFN-γ) were dynamically monitored. Results: On D1 of admission, the numbers of peripheral blood CD3+ T, CD4+ T, CD8+ T, and NK cells were significantly lower than the normal range. With the improvement of the disease, the numbers of CD3+ T, CD8+ T and CD4 + T cells gradually recovered and showed a linear growth trend (linear fitting equation: Y=18.59X+109.4, P < 0.05). On D2 of admission, the patient's IL-6 and IL-10 levels were significantly higher than normal values, IFN-γ was at a normal high value, and then rapidly decreased; IL-2, IL-4, and TNF-α were all in the normal range. On the D6 and D7, the IL-6 and IL-10 decreased to the normal range for the first time. On the D18, the sputum virus nucleic acid test was negative for the first time, and the fecal virus nucleic acid test was still positive; on the D20 the sputum and fecal virus nucleic acid test were both negative. On D34, the patient recovered and was discharged. At the discharge the muscle strength score of the patient was 44 and the daily life ability evaluation was 90. Conclusion: In the absence of effective antiviral drugs, early use of appropriate doses of corticosteroids in critically ill patient with COVID-19 can quickly alleviate inflammatory response and improve clinical symptoms, however, it may reduce the number of T cells, and to adjust the dose in time is necessary.