Objective: To analyze the effectiveness of buttress plates and cannulated screw in the treatment of type IV large posterior malleolus fractures of ankle with supination-external rotation via the posterolateral approach. Methods: Between August 2016 to August 2018, 72 patients (41 males and 31 females; age from 21 to 67 years, mean age 42.86 years) with type IV large posterior malleolus fractures of ankle with supination-external rotation were treated by cannulated screw or buttress plate fixation by the posterolateral approach. The posterior malleolus fractures were fixed with buttress plate, including 23 males and 17 females. The posterior malleolus fractures were fixed with cannulated screw, including 18 males and 14 females. The operative time, perioperative blood loss, the bone healing time and the postoperative complications were assessed. Ankle joint function was evaluated according AOFAS scoring system. Results: All the patients were followed for 12–16 months (13.13 months on average). After the follow-up period, no incision dehiscence, delayed fracture healing, non-healing phenomenon was observed. According to the AOFAS scoring system, the outcomes in buttress plate group were rated as excellent in 15 cases, good in 19, fair in 5, and poor in only 1 case, for an excellent and good rate of 85.0%. And the outcomes in cannulated screw group were rated as excellent in 11 cases, good in 15, fair in 4, and poor in only 2 case, for an excellent and good rate of 81.3%. There were significant differences between the two groups in operative time, perioperative blood loss and the AOFAS score (P<0.05). The complication formation rate (buttress plate group 7.5%, cannulated screw group 12.5%) and the bone healing time for two groups were no significant differences (P>0.05). Conclusion: A posterolateral approach with buttress plate internal fixation could be more effective technique for reduction and fixation of type IV large posterior malleolus fragments of ankle with supination-external rotation.