Introduction: Ankle braces can effectively decrease the incidence of recurrent ankle sprain; however, whether the brace can decrease the severity of sprain and its related mechanism during sprain remain unknown. Methods: Twenty-two patients with functional ankle instability (FAI) (12 males and 10 females) and 16 healthy subjects (8 males and 8 females) were enrolled in this study. All of the subjects walked on a custom-built tilting platform that offered a 30 degrees inversion (IV) to mimic the IV of ankle sprain. We collected the kinematic and surface electromyography data of patients with FAI with or without ankle brace and normal controls 6 times. Results: The FAI without brace group showed significantly higher maximum IV angles and average IV velocities than the control group (P < .001). The FAI with brace group revealed significantly lower maximum IV angles and average IV velocities than the FAI without brace group (P < .001); this group also showed significantly higher maximum external rotation (ER) angle and average ER velocities than the FAI with brace (P < .001) and control (P < .001) groups. The FAI with brace group indicated significantly lower average EMG(Prep) (P = .047), EMG(Tilt) (P = .037), and EMG(afterTilt) (P = .004) of the peroneus longus than the FAI without brace group. Conclusions: The ankle brace could effectively decrease IV angles and their velocities and increase ER angles and their corresponding velocities during ankle sprain in patients with FAI. It could also decrease the activity of the peroneus longus muscle during ankle sprain.