〔Abstract〕 Objective To explore the key points of differential diagnosis of Pneumocystis carinii pneumonia (PCP) and 2019 novel coronavirus pneumonia (COVID–19) characterized by multiple ground glass lesions in both lungs. Methods Retrospective analysis of the clinical characteristics (epidemiological history, symptoms, laboratory examinations and imaging performance) of fi ve COVID–19 patients with multiple ground glass lesions diagnosed in our hospital from January 2020 to April 2020, and through a case of suspected COVID–19 but eventually diagnosed as acquired immunodefi ciency syndrome (AIDS) secondary PCP cases, to explore the points of the differential diagnosis of the twodesease. Results (1) Epidemiological characteristics: Most patients with COVID–19 have a history of traveling in Wuhan, and the population is generally susceptible; While patients with secondary AIDS secondary to PCP generally have a history of stagnation, and the general infection is immunodefi ciency. (2) Clinical manifestations: Fever and dry cough are the main clinical manifestations. COVID–19 has an acute onset. Ordinary and mild patients generally have no diffi culty in breathing. While PCP generally has a subacute onset, progressively increased dyspnea, and severe hypoxemia. (3) Laboratory examination: the two desease often show normal white blood cells, decreased lymphocytes, and increased infl ammation indicators. (4) Imaging features: both chest CT can show multiple ground glass lesions of the lungs, and the distribution of COVID–19 lesions is generally under the external pleura.While PCP is a ground glass shadow with the hilum as the center, and the pleura is rarely involved. (5) The outcome of treatment: COVID–19 has no special drug treatment, mainly symptomatic support treatment, and PCP has a better treatment effect. Conclusion PCP and COVID–19 have similar clinical symptoms and imaging findings, but there are still some differences in epidemiological characteristics, immune status, the onset process, chest disease distribution, and treatment outcomes.