BACKGROUND: As the end bearing joint of the human body, the ankle joint bears the top-down pressure of the body, which leads to the ankle joint is easy to be damaged in the movement, can induce functional ankle instability, which negatively affects daily life. The study of lower extremity biomechanics in patients with functional ankle instability during counter movement jump is of great significance for scientific training, prevention of ankle injury, and clinical rehabilitation after injury. OBJECTIVE: To investigate the kinetics and kinematics of lower limbs in the longitudinal jumping of functional ankle instability population. METHODS: From March to September 2023, 15 male patients with functional ankle instability and 15 healthy people, aged 22-28 years old, were recruited in Soochow University. All subjects completed counter movement jump experiment. Vicon infrared high-speed motion capture system and Kistler threedimensional force measuring table were used to simultaneously collect the lower limb kinematics and kinetics indexes of the two groups of subjects at the take-off stage of counter movement jump, the instant off the ground, the initial landing moment and the peak moment of vertical ground reaction force. RESULTS AND CONCLUSION: (1) At the instant off the ground, the affected side of the functional ankle instability group showed smaller knee internal rotation moment (P=0.020) and smaller ankle internal rotation moment (P=0.009) compared with the affected side of the healthy control group. (2) At the moment of landing, the affected side of the functional ankle instability group showed a smaller hip flexion angle than the affected side of the healthy control group (P=0.039). Compared with the healthy control group, functional ankle instability group showed smaller hip abduction angle (P=0.022), smaller knee varus angle (P=0.010), larger knee external rotation angle (P=0.021), smaller ankle varus angle (P=0.004), and smaller external ankle rotation angle (P=0.008). (3) At the peak of vertical ground reaction force, functional ankle instability group showed a smaller ankle varus angle than healthy control group (P=0.044). (4) The results showed that the lower limb biomechanical characteristics of the patients with functional ankle instability were abnormal compared with the healthy people during counter movement jump, which mainly showed the changes of the kinematics and kinetics indexes of the lower limb joints in the sagittal plane and the frontal plane at the moment of lift-off and landing. These changes reflect that people with functional ankle instability adopt rigid take-off and landing patterns when performing counter movement jump, tend to transfer the load of the affected ankle joint to other joints of the lower limb, and show compensatory phenomenon of the healthy lower limb. Therefore, detection and correction of abnormal biomechanical features should be a part of rehabilitation training for those with functional ankle instability. [ABSTRACT FROM AUTHOR]