Abstract: Objective To improve the understanding of retroperitoneal fibrosis. Methods A patient with retroperitoneal fibrosis, an IgG4-related sclerosing disease was reported.Its clinical characteristics, diagnosis and treatment were analyzed.The patient visited our department due to left lumbar pain for 1 year and left lower extremity numbness for 3 months.Abdominal CT showed an occupying disease in retroperitoneal iliac fossa and unclear left kidney.The retroperitoneal tumor and the involved sigmoid colon were excised.Fistulization was performed for descending colon with the involved left ureter partly removed. Results The patient was recovered and discharged.The patient was diagnosed with chronic inflammation of sigmoid colon tumor and mucositis with fibrous tissue hyaline degeneration in colon serous layer accompanying massive inflammatory cell infiltrative nodules. Conclusion Progressive retroperitoneal fibrous tissue proliferation due to unknown causes is a local manifestation of systemic immune system diseases.Surgical excision is a reliable procedure for the removal of compression caused by the lesion, followed by glucocorticoid hormone and immunosuppressive therapy.