The invasion of the pleural membrane by a malignant pleural tumor can lead to the production of malignant pleural effusion (MPE), resulting in the symptoms of dyspnea, and some patients have cough, sputum and other symptoms, which are easily confused with pneumonia. In this case, the initial diagnosis of the patient is pneumonia, and the final diagnosis is pneumonia combined with pleural malignancy. Therefore, if the patient has unexplained symptoms of bloody pleural effusion, it is necessary to examine for malignant tumors and should actively perform thoracentesis and drainage, look for malignant cells in the pleural effusion cell precipitation, evaluate the nature of pleural effusion, conduct pleural biopsy tissue examination, and determine the type and source of lung malignancy by the combined application of cell block technology and immunohistochemistry. Take the cytological examination results in pleural effusion seriously, and finally, surgical or immunotherapy can be performed.