For golf swing, the soft tissue structure resisting joint compression and internal rotation of the knee at low flexion angle may be susceptible for a lead knee injury. Therefore, anterior cruciate ligament (ACL) rupture is one of the potential injuries that may occur from repeated stress during golf. The current study was purposed to investigate the biomechanical factors that lead to high ACL load in the lead knee during golf swing. The joint kinematic data of the lead leg and trunk, joint kinetic data of the lead knee, ground reaction force, and the external knee moments were compared between the low and high ACL loading groups. The results demonstrated an increased amount of frontal plane moment arm and external knee adduction (varus) moment just after ball impact for the high ACL loading group. These observations were associated with a characteristic difference in the upper body motion and were the main contributors to the elevated ACL force of the lead knee. The mechanism that generates a high amount of ACL loading during golf swing, which involves the application of external knee adduction moment just after ball impact, may differ from conventional non-contact ACL injury mechanisms that associated with dynamic valgus loading during injury circumstance.