The serous cavity mentioned in this consensus refers to the pericardial cavity, pleural cavity, and peritoneal cavity. Malignant serous effusion refers to the detection of malignant cells in the serous effusion through exfoliate cytology or the diagnosis of malignant tumor by serosal tissue biopsy. Malignant serous effusion is a special manifestation of tumor metastasis, and it is one of the common complications of patients with advanced malignant tumors. Large amounts of malignant serous effusion can affect the quality of life seriously and be even life-threatening. The treatment concepts for malignant serous effusion are obviously different between China and European and American. Currently, diagnosis and treatment guidelines or norms are insufficient worldwide. The existing expert consensus on the diagnosis and treatment of malignant serous effusion in China was published in 2014. It does not include pericardial effusion and ascites effusion, and some treatment strategies (such as talcum pleurodesis) are not widely used in clinical practice. This consensus mainly includes three parts. The first is to introduce the global treatment status and the main treatment concepts in European and American countries, as well as the progress of treatment in China. The second part gives a detailed introduction to the diagnosis process, including medical history, clinical manifestations, imaging examinations, and tumor marker detection. The focus is on the exfoliative cytology and serosal biopsy methods, including the application of paraffin-embedded section technology of cell blocks in molecular genetic testing that has emerged in recent years. The third part discusses the treatment process, including the principles of local and systemic treatment. In terms of local treatment, the indications, main drug recommendations, drug dosage, infusion time, prevention and treatment of adverse reactions, and treatment methods for special types of malignant effusion are introduced in detail for common fluid drainage or drainage combined with drug infusion therapy. In terms of systemic therapy, clear guidance is also given to the principles of systemic therapy and the choice of systemic drug delivery. In the last part, the indications and operating procedures of external thermo-circulation perfusion chemotherapy are explained briefly. At present, although there is a large amount of Chinese literature on the diagnosis and treatment of malignant serous effusions, the quality of the literature is not high, let alone the level of evidence. However, this consensus has fully considered the actual conditions of different levels of medical institutions, and has universal applicability, strong practicability, and maneuverability.