Background: Pneumocystis jirovecii pneumonia (PCP) is a common illness of the patients receiving long-term immunosuppressive treatment for which therapy is currently available. Early and prompt diagnosis of this pulmonary infection is a matter of great practical importance. Clinical presentation and radiology are not enough to confirm a diagnosis of PCP. Microscopic examination is available.Case presentation: Here we report a 63-year-old man with nephrotic syndrome,who was transferred to ICU due to unexplained aggravation of infection. The cause of pulmonary infection was identified by Rapid on-site evaluation (ROSE) finally and targeted treatment was given.Conclusions: ROSE can be a diagnostic approach for PCP, which is suitable for clinicians.