Objective By comparing the differences in biomechanical characteristics of the lower limbs between individuals with and without Chronic Ankle Instability (CAI) under unanticipated and anticipated jumping, we can provide practical references and ideas for the prevention and treatment of recurrent ankle sprains. Methods Thirty participants were recruited, including 15 individuals with CAI and 15 healthy participants. All participants completed unanticipated and anticipated jumping tests in random order, with a 1-week interval between the two tests. Kinematic and kinetic data were collected at the moment of touchdown and during the early landing phase. A 9-lens infrared high-speed motion capture system (Vicon, Nexus, T40, 200 Hz) and a Kistler 3D force platform (1000 Hz) were used to synchronously collect kinematic and kinetic data. Results At the moment of touchdown, knee flexion angle was significantly greater in unanticipated jumping than in anticipated jumping (P=0.009), while ankle eversion angle was notably lower (P=0.043). During the early landing phase, unanticipated jumping showed significantly greater peak hip flexion and abduction angles, as well as knee flexion (P=0.038, P=0.036, and P=0.012), while peak ankle dorsiflexion and eversion angles were significantly lower (P=0.001, P=0.010) compared to anticipated jumping. Additionally, peak hip abduction moment during unanticipated jumping was significantly higher in individuals with CAI than in anticipated jumping (P=0.028). Conclusions Unanticipated jumping reduced ankle dorsiflexion and eversion angles in individuals with CAI, putting the ankle in an open, sprain-prone position; Individuals with CAI compensated proximally by increasing hip flexion, abduction, knee flexion angles, and hip extension moments to stabilize the ankle.