Background and purpose: Blood biochemistry, ascites test and imaging diagnosis have low sensitivity for the diagnosis of abdominal neoplasma. And also exploratory laparotomy is not suitable for late stage patients. Real-time three-dimensional(3D) ultrasound is a new method in the diagnosis of celiac disease. We aimed to explore the application of this new tool in the diagnosis of abdominal cavity metastases of malignant tumor. Methods: Twenty-eight patients with malignant tumor preoperatively underwent both 2D and real-time 3D ultrasound examination The results were compared to pathological and clinical outcomes. Any one of those signs such as abdominal cavity mass, abdominal cavity lymphadenectasis, thickening of the peritoneum, nodules over the peritoneal and abdominal organs surface were considered as abdominal cavity metastases. Results: Twenty-five of them were pathologically confirmed to have abdominal cavity metastases. The diagnosis correlation rate of 2D and real-time 3D in the diagnosis of abdominal cavity metastases of malignant tumor were 42.86% and 92.86%, respectively. The sensitivity of these two methods was 37.5% and 95.83%, respectively. The speificity of two methods was 75.0%. The detection rates of the two methods were 16% and 96%, respectively. There were highly significant differences between two groups (P<0.001).Conclusion: Real-time 3D ultrasound had more value in the diagnosis of abdominal cavity metastases of malignant tumor in terms of sensitivity, specificity and positive detection.