Abstract: The purpose of this study was to investigate the effects of unanticipated condition on knee joint biomechanics, and to discover the potential risk of ACL injuries during sidestep-cutting among female football players. Methods:Female football players were recruited in this study. Participants were asked to perform the sidestep-cutting technique during anticipated and unanticipated conditions. Kinematics data was recorded by Vicon and kinetic data was recorded by one force plate (Kistler). Knee joint angles, moments, and lateral Ground Reaction Force (GRI) were analyzed. Paired t-test was performed to compare the kinematics and kinetics variables between anticipated and unanticipated conditions. Results:1) Knee joint was varus in anticipated condition, while was at valgus in unanticipated condition. 2) Maximum knee flexion angle, maximum knee valgus angle and maximum knee internal rotation angle were bigger in unanticipated condition, compared with anticipated condition during sidestep-cutting (P < 0.05). 3) The main effect was from valgus and external rotation moment in unanticipated condition while it was from varus and internal rotation moment in anticipated condition. 4) The lateral GRI in unanticipated condition was lager than the anticipated condition (P < 0.05). Conclusion:Compared with anticipate condition, the valgus status of knee joint and bigger valgus and internal rotation angle of knee joint at buffer phase might be the major biomechanics mechanism of bigger ground reaction force and natural transition of knee moment at unanticipated condition during side cutting. It indicated that side cutting movement might increase the risk of ACL injury at unanticipated condition.