Objective:To investigate the application value of mixed reality (MR) technology in reconstruction of soft tissue defect of extremities with free anterolateral thigh flap(ALTF).Methods:From December 2019 to November 2021, a retrospective analysis was performed on 10 patients who had undergone ALTF reconstruction of soft tissue defects in extremities in Department of Orthopaedics, the First People's Hospital of Yunnan Province. Four patients had the defects in hand and 6 patients in foot and ankle. For the 6 patients in emergency surgery, the time from injury to admission was 4.0-15.0 hours, with an average of 7.3 hours. Four patients with soft tissue defects caused by chronic infection and ulcers were given debridement, and the soft tissue defects were reconstructed by flap transfer at the second stage. The defect area were from 8.0 cm×5.0 cm to 22.0 cm×8.0 cm. Preoperatively, 3D bone-vessel-flap model was established based on the lower extremity CTA scans. Intraoperatively, MR technology was used to project the 3D model on the flap donor site to observe the virtual profile of vessel shape in real time, to locate the perforator and the course of the perforator, and observe the consistency between the virtual image and the actual anatomy of the perforator. The appearance, texture and colour of the flap were recorded at the last follow-up. Hand function was evaluated by the total activity movement (TAM), and foot and ankle function was evaluated by the American Orthopaedic Foot and Ankle Society (AOFAS).Results:The position location and course of perforator vessels were reconstructed successfully in all patients before surgery. The MR technology was used to locate the perforator, and the course of the virtual perforator was consistent with the actual anatomy, and the matching reached 100%. The length of vascular pedicle measured before surgery was at 11.02 cm±1.37 cm. And that measured during surgery was at 11.21 cm±1.23 cm ( P=0.748, t=-0.326). The difference was not statistically significant ( P>0.05). The flap area was at 9.0 cm×6.0 cm to 23.0 cm×9.0 cm. The donor site was sutured directly in one stage. All patients were entered postoperative followed-up for 1 to 24 months, with an average of 13.5 months. All the flaps survived after surgery. The flap with good appearance, colour and texture, and only one linear scar was left in the donor site. According to the TAM of the hand function, 3 cases were excellent and 1 was fair. Foot and ankle function were evaluated according to the AOFAS, 5 cases were in excellent and 1 was good. Conclusion:MR technology applied to the surgery of ALTF can locate the course of the flap vessels in real time, guide the operation, improve the operation efficiency and reduce the risk in surgery.