Abstract： Objective： To observe the clinical effect of Peiyuan Kangai Decoction combined with external application of Natrii Sulfas and abdominocentesis on malignant peritoneal effusion，and its effects on the levels of tumor markers and immune T lymphocytes. Methods： A total of 106 cases of patients with malignant peritoneal effusion treated at Ruian Hospital of Traditional Chinese Medicine from June 2020 to December 2023 were selected and divided into the control group（50 cases）and the treatment group（56 cases）using the random number table method. The control group received conventional abdominocentesis and standard western medical treatment，and the treatment group was additionally given the oral administration of Peiyuan Kangai Decoction and external application of Natrii Sulfas based on the control group's treatment. After four weeks of treatment， clinical effects， traditional Chinese medicine（TCM） syndrome scores，tumor markers [squamous cell carcinoma antigen（SCC），cancer antigen 125（CA125），cytokeratin 19 fragment（CYFRA21-1）]，immune T lymphocytes（CD4 + ，CD8 + ，CD4 + /CD8 + ）levels，and adverse reactions were compared between the two groups. Results：The disease control rate was 91.07%（51/56）in the treatment group and 74.00%（37/50）in the control group，with a significant difference between the two groups（P＜0.05）. After treatment， the TCM syndrome scores for abdominal distension，soreness and weakness of waist and knees，and fear of cold and cold limbs in the two groups were decreased when compared with those before treatment（P＜0.05），with the treatment group showing lower scores than the control group（P＜0.05）. After treatment， the levels of SCC， CA125， and CYFRA21-1 in the two groups reduced when compared with those before treatment（P＜0.05），with the treatment group showing lower levels than the control group（P＜0.05）. After treatment，the levels of CD4 + and CD4 + /CD8 + in the two groups were increased when compared with those before treatment，and CD8 + levels were decreased when compared with those before treatment（P＜0.05），with the treatment group showing higher levels of CD4 + and CD4 + /CD8 + and lower CD8 + level than the control group（P＜0.05）. The total incidence of adverse reactions was 7.14%（4/56）in the treatment group and 22.00%（11/50）in the control group，with a significant difference between the two groups（P＜ 0.05）. Conclusion：Peiyuan Kangai Decoction combined with external application of Natrii Sulfas and abdominocentesis has a significant curative effect in treating malignant peritoneal effusion. It can significantly improve TCM syndromes and immune function and reduce tumor marker levels，with fewer adverse reactions.