Malignant pleural effusion is distinguished from benign pleural effusion by the appearance of the effusion, the composition of the effusion, the outcome after treatment, and the prognosis. 1. From the appearance point of view, malignant pleural effusion is mostly bloody or turbid with impurities or flocculent material, etc., while benign pleural effusion is mostly transparent and bright yellowish fluid. 2. Malignant pleural effusion can be found in malignant tumor cells, while benign pleural effusion usually does not have tumor cells, but may have neutrophils, lymphocytes and other inflammatory cells. 3. Malignant pleural effusion is usually treated with chemotherapeutic drugs, targeted drugs and immune drugs, and malignant effusion is easy to generate repeatedly. Generally, benign pleural effusion will disappear after anti-inflammatory and protein supplementation treatments. 4. After treatment of benign pleural effusion, the effusion will be reduced to disappear, and its prognosis is better than that of malignant effusion.