Objective To explore the protective mechanism of ankle braces in patients with functional ankle instability (FAI), so as to provide references for the selection of ankle braces. Methods Fifteen FAI patients randomly wore semi-rigid and elastic ankle braces or wore without braces for walking and running at the selfselected speed. The lower extremity biomechanical parameters were collected by infrared motion capture and three-dimensional (3D) force plate. The 3 × 2 repeated two-way ANOVA was used to test the effects of ankle brace and movement patterns on biomechanical parameters of the lower extremity. Results There was no interaction between ankle braces and movement patterns on all indexes in the study (P>0. 05). Regardless of movement patterns, compared with no ankle brace support, the elastic ankle brace significantly reduced peak ankle inversion angle, inversion angular velocity and energy absorption (P<0. 05) for ankle joint, and increased ankle eversion moment ( P < 0. 001). The semi-rigid ankle brace increased peak ankle inversion angle and inversion angular velocity ( P < 0. 05). In addition, the elastic ankle brace reduced the knee internal rotation moment at the landing moment and the peak knee external rotation moment (P<0. 05). Conclusions Compared with no ankle brace support, the elastic ankle brace reduced the ankle inversion angle, inversion angular velocity and energy absorption, increased the ankle eversion moment and played a role in preventing ankle sprain. For FAI patients wearing semi-rigid ankle brace, regular attention should be paid to the risk of chronic ankle injury. Overall, the protective effect of elastic ankle brace may be more prominent and does not cause knee joint function compensation, which can be used as an effective measure to prevent ankle sprain in FAI patients.