The child, Xiao Li, was 9 years old. He was confirmed to have osteosarcoma of the lower femur. After chemotherapy, the tumor segment was resected and fixed with an external fixation brace. 3 months later, the severed end healed, and 6 months later, the external fixation brace was removed and the prosthesis was installed and functional exercise was started, and the child quickly resumed walking. The patient has now been followed up for 9 years, and there is no obvious recurrence or metastasis, and the child has also increased from 135cm at the time of treatment to 176cm. The treatment effect is very satisfactory. Conclusion: For malignant bone tumor, life is the first, limb is the second, and function is the third. This procedure maximizes the length of the limb and restores the function of the knee while removing the whole tumor as much as possible.