To analyze the Clinical effect of retroperitoneal malignant tumors treated by ultrasound-guided percutaneous radioactive iodine-125 seed Implantation. The clinical data, treatments and follow-up results of 26 patients with retroperitoneal malignant tumors diagnosed between April 2012 and June 2015 were analyzed retrospectively. Among these patients, 8 were males and 18 were females. Patients' ages ranged from 29 to 82 years (median 51.7 years). Pain relief,tumor local control, overall survival and complications were observed post-operation. In our 26 patients’ group, 2 patients had no pain, 6 patients had mild pain, 10 patients had moderate pain, and 8 patients had severe pain before implantation. 1 month after implantation, we observed that 18 patients completely remission, 4 patients partial remission, and 4 patient no response. The rate of pain relief in 26 patients (CR+PR) was 84.6%.; After 2 months’ of implantation, all 26 patients received CT scan and compared with pre-operation CT images. 3 patients’ tumors completely remission, 20 patients’ partial remission, 2 patients with stable disease and 1 patient with progressive disease. The rate of tumor local control in 26 patients (CR+PR) was 88.4%.The follow-up period was 3-36 months. Median survival time was 11 months. After 36 months follow-up, there were 6 patients still surviving. The 1 year and 2 years overall survival rates were 46% and 27%, respectively. The mean survival time was 9.4 months in 5 patients who were diagnosed pancreatic cancer, significantly higher than the natural survival time of 3-6 months. Patients in this group had no severe complications. Ultrasound-guided percutaneous radioactive iodine-125 seed implantation can effectively relieve pain of retroperitoneal carcinoma; it can effectively local control the development of tumors and relieve patient state; it is a safely method to treat nonresectable retroperitoneal malignant tumors.