For the diagnosis of pleural effusion, the following process is generally followed clinically: 1. The presence of pleural effusion is determined through a series of clinical and ancillary examinations, which may include physical examination, ultrasonography or imaging examinations; 2. The nature of pleural effusion is determined through laboratory tests related to pleural effusion, including exudate and leaky fluid, which relies on thoracentesis for pleural fluid This relies on thoracentesis for pleural fluid; 3. A very critical step is to conduct the examination of the etiology of pleural effusion, and there is a clinical process of simultaneous diagnosis and differential diagnosis in part of the etiological examination. The primary differentiation is between benign and malignant pleural effusions, and there are many ways to complete the clinical differentiation. For pleural effusions of different nature or different diseases also determines the direction of patient treatment and the final prognosis of the patient.