Objective: To investigate the effect of preoperative radiotherapy in the patients with primary retroperitoneal soft tissue sarcoma difficult to resect or unresectable and recognize the results of radiotherapy associated with resection and the effect of treatment on prognosis. Methods: It was analyzed retrospectively with 44 cases of primary retroperitoneal soft tissue sarcoma difficult to resect or unresectable in Department of Radiation Oncology, Zhongshan Hospital from May 2017 to February 2022. Intensity modulated radiation therapy (IMRT) and helical tomotherapy (TOMO) were used to treat the patients, and the effects of conversion to resection treatment for patients with the prognosis and complications were analyzed. Results: A total of 44 cases received radiotherapy including 30 cases with IMRT and 14 cases with TOMO. The adverse events of radiotherapy were grade 1 to 2. After preoperative radiotherapy, 12 cases achieved partial response, 26 cases remained in stable condition and 6 cases progressed. There were 26 cases treated by surgical excision after radiotherapy with the rate of conversion to resection 59.1% (26/44). The 1- and 3-year survival rates of the cases were 93% and 72% respectively. Conclusions: The preoperative radiotherapy could have improve the conversion rates of resection for the patients with primary retroperitoneal soft tissue sarcoma difficult to resect or unresectable. The better postoperative survival was gotten with mild adverse events.