Abstract: Objective To study the application of anterolateral approach to ankle joint under prone position to assist medial incision in the treatment of trimalleolar fracture. Methods Thirty-nine patients with trimalleolar fracture admitted to the Department of Orthopedics, Chuzhou First People's Hospital, Anhui Province from September 2015 to November 2018 were selected. According to Lauge-Hanse classification, 24 cases were supination-abduction type, 10 cases were supination-abduction type, 3 cases were supination-abduction type, 1 case was supination-adduction type, and 1 case was vertical compression type. All patients were treated with medial incision internal fixation assisted by lower posterolateral approach in prone position. The operation position is prone position. The lateral malleolus is firstly restored and fixed through the posterolateral approach. After the restoration of lateral malleolus, it is relatively easy to restore and fix the posterior malleolus due to the stretching effect of the posterior joint capsule. During the operation, the stability of the lower tibiofibular syndesmosis is checked by "hook test". If necessary, the lower tibiofibular syndesmosis is fixed by 3-layer cortical bone screws at the lateral malleolus. The internal fixation sequence is the lateral malleolus and the posterior malleolus. Finally, the medial incision is assisted to restore and fix the medial malleolus, and the steel plate is placed at the posterior side or lateral side of During the operation, the anatomical reduction and internal fixation of trimalleolar fracture were confirmed by screen monitoring of C-arm X-ray machine. The fracture reduction, healing and ankle joint function were observed after operation. Results All 39 patients were followed up for an average of 13 (4-26) months. Baird-Jackson ankle joint scoring system was used in all six months after the operation, with 10 cases (25.6%) with more than 95 points, 23 cases (59.0%) with 91-95 points, 5 cases with 81-90 points, and 1 case with less than 80 points, with a total excellent and good rate of 84.6%. Conclusion Prone lower posterolateral approach assisted with medial incision for trimalleolar fracture is simple and convenient to operate. It can reduce and fix the posterior malleolar fracture block under direct vision. At the same time, lateral malleolus and medial malleolus can also be well exposed, reduced and fixed. It has definite clinical effect and is suitable for trimalleolar fracture surgery.