摘要： Objective: To investigate and discuss the clinical value of positron emission tomography-computed tomography (PET-CT) combined with ultrasound in detection of primary tumors in patients with malignant ascites (MA). Materials and Methods: A total of 122 malignant tumor patients with ascites as the initial symptom and 48 patients with benign ascites were enrolled in this study. All patients underwent PET-CT and abdominal B-ultrasound examinations. The corresponding specificity, sensitivity, accuracy rate, positive predictive value, and negative predictive value of PET-CT, abdominal B-ultrasound, and combined detection group were recorded, respectively, with pathological findings as the gold standards. Statistical Product and Service Solutions 17.0 software was used for statistical analysis. p < 0.05 suggested that the difference was statistically significant. Results: The detection rate of primary foci through PET-CT was 79.5%, of which the detection rate of primary foci of MA derived from gastric cancer was the highest. The detection rate of primary foci through B-ultrasound was 62.5%, which is the highest for MA derived from ovarian cancer. B-ultrasound had the highest specificity in diagnosing the primary foci of MA (73.2%), PET-CT had the highest sensitivity in diagnosing the primary foci of MA (91.7%), and PET-CT combined with abdominal B-ultrasound had the highest sensitivity and accuracy in diagnosing the primary foci of MA (98.1% and 89.1%, respectively). The diagnostic accuracy rate of B-ultrasound was the highest in detecting tumors >5 cm (77.0%), whereas that of PET-CT was the highest in detecting tumors of 3–5 cm (84.2%). Conclusion: The PET-CT combined with ultrasound is conducive to improving the diagnostic efficiency for primary tumors in patients with MA.