Objective To investigate the sonographic characteristics of the primary retroperitoneal sarcoma. Methods Ultrasonographic features of 47 patients with primary retroperitoneal sarcoma confirmed by biopsy or surgical pathology were retrospectively analyzed. Results Among all 47 primary retroperitoneal sarcomas, single lesion was found in 42 cases, while multiple lesions were found in 5 cases. Pathology confirmed 29 cases of liposarcoma, 13 cases of leiomyosarcoma, 4 cases of rhabdomyosarcoma, and 1 case of ewing’s sarcoma. The diameters of all tumors were 2.0-55.0cm, average 14.3±9.6cm. There were 32 cases presented hypoechoic, 8 cases with hyperechoic and 7 cases with complex echoic. Sixteen tumors were inhomogeneous, 12 cases were accompanied with no-echo areas, and 5 cases with calcification in the tumor. Well-defined boundary of tumors was found in 26 cases, and ill-defined boundary of tumors were found in the other 21 cases. Regular shape of tumors was found in 15 cases, while the other 32 cases were presented as irregular shape. Color Doppler flow imaging (CDFI) showed abundant tumor blood flow signals in 35 cases, and no obvious blood detected in the other 12 cases. In addition, 18 tumors invaded peripheral organs with different degrees, four cases accompanied with retroperitoneal lymph node metastasis, and 1 case with tumor thrombosis in bilateral common iliac vein and inferior vena cava. Conclusion Primary retroperitoneal sarcomas are usually present as large size, and grayscale ultrasound usually showed as well-defined hypoechoic masses with irregular shape in the retroperitoneum. Necrotic liquefaction areas or patchy calcification can be seen in some tumors, and abundant tumor blood flow signals can be detected in majority of retroperitoneal sarcomas on CDFI.