Supination external rotation (SER) type ankle fracture is the most common ankle fracture in the Lauge-Hansen classification and is often accompanied with syndesmotic injury. However, the mechanism of this injury is indistinct and a suggestive role can be given by preoperative imaging. This study was to preoperatively predict whether SER type ankle fractures are accompanied with syndesmotic injuries by the means of lateral malleolus fracture mapping. One hundred and forty-eight patients diagnosed with SER type ankle fractures were retrospectively enrolled in this study. The baseline data were collected and computed tomography data were reconstructed in 3-dimensional (3D) model. Patients were divided into stable and unstable groups according to intraoperative Cotton test and whether the inferior tibiofibular screw was placed. All fracture lines were superimposed on the ankle template to create a fracture map, and the data on the fracture map were further measured. Logistic regression was conducted to identify relevant factors and the cutoff values were given using receiver operating characteristic curves. Forty-one patients were enrolled in the unstable group and 107 patients were enrolled in the stable group. The lateral malleolus fracture lines of the unstable group were higher and steeper than that in the stable group on lateral and posterior views. The fracture height of the posterior cortex and peak height were the significant contributing factors, and the cut-off values of posterior cortex, peak height and inclination angle were 40.35 mm (sensitivity: 78%, specificity: 82%), 55.34 mm (sensitivity: 85%, specificity: 70%) and 55.6° (sensitivity: 66%, specificity: 86%), respectively. In general, when the fracture lines of the lateral malleolus were high and steep, it was usually indicative of a syndesmotic injury and can be predicted by the preoperative 3D reconstruction of fracture height of posterior cortex, peak height and inclination angle. If the cut-off values of these indicators are exceeded, the syndesmotic injuries may be presented and need to be verified in the intraoperative Cotton test to decide whether to insert an inferior tibiofibular screw.