Retroperitoneal fibrosis refers to a group of diseases characterized by hyperplasia of the fibrosclerotic tissues in the retroperitoneal space, which can compress the surrounding ureters and inferior vena cava and cause serious complications such as aortic aneurysm, renal failure, and even death. The lesion is diffuse and difficult to resect. corticosteroid is the first-line medication, but the recurrence rate of the disease is high, especially after dose reduction of corticosteroid. Therefore, the combined use of immunosuppressants is very important in preventing disease recurrence and reducing the toxic and side effects of long-term corticosteroid. Sirolimus plays dual roles in inhibiting lymphocyte activation and fibroblast proliferation. It is inferred from its mechanism that sirolimus is a good potential treatment option for idiopathic retroperitoneal fibrosis. Therefore, we conducted this RCT on patients with idiopathic retroperitoneal fibrosis to determine the efficacy and safety of sirolimus.