Objectives In children with distal femoral osteosarcoma (San-Julian type III), combined with tibial epiphysis preservation can prolong the early clinical study of tumor prosthesis reconstruction. Methods From October 2017 to October 2018, biopsy pathology confirmed osteosarcoma in our hospital. Neoadjuvant chemotherapy was given for 1 to 2 cycles before surgery (all chemotherapy regiments were APMI, which was administered sequentially). After chemotherapy, the sensitivity to chemotherapy and distant metastasis were evaluated by reexamination of DR,CT,MRI and bone scan. After neoadjuvant chemotherapy, all the children were classified as san-julian type III according to imaging classification. There were 6 children in total, including 2 males and 4 females. They were 7 to 12 years old, with an average age of about 9.7 years. There were 3 cases on the left and 3 cases on the right. In our department, all patients received the whole excision of joint capsule combined with preservation of tibia epiphysis, which can prolong the reconstruction of tumor type artificial prosthesis. After the operation, the children were instructed to perform functional exercise. The stitches were removed 2 weeks after the wound, and the postoperative standardized chemotherapy was given after another week, and follow-up was conducted. Results The operating time of the 6 children was 2.5h, 3h, 3h, 3.2h, 3.5h and 4h, respectively, with an average of 3.2h. Intraoperative blood loss was 600 ~ 1200ml, and the average blood loss was 850ml. The follow-up time of 6 children was 9-21 months, and the average follow-up time was 15 months. According to the Musculoskeletal Tumor Society (MSTS), functional scores ranged from 18 to 27, with a median score of 23, at 3 months after surgery. In the last follow-up, 4 patients had knee flexion angles of 90 to 110 degrees, and 2 patients had knee flexion angles of 110 to 130 degrees, all of which could reach 180 degrees. The difference in length of both lower limbs was less than 1.5cm in 5 cases, and about 2.5cm in 1 case. No local recurrence or pulmonary metastasis occurred during the follow-up period. Conclusion By strictly controlling surgical indications, for children with type III of the san-julian classification of distal femoral osteosarcoma, extraarticular capsulectomy combined with tibial epiphysis preservation can provide a safe and effective surgical method for prolongation of tumor type artificial joint prosthesis reconstruction.