BACKGROUND: Physical fatigue in soccer has been considered a risk factor for anterior cruciate ligament (ACL) injury, while mental fatigue could lead to decision-making errors and induce injuries. However, there is a lack of research on the combined effects of mental fatigue and decision making on lower limb biomechanics. HYPOTHESES: (1) Mental fatigue leads to a decrease in trunk flexion angle and knee flexion angle when touching the ground, as well as an increase in knee valgus moment and horizontal backward ground-reaction force (GRF) under unexpected conditions, which may lead to an increase in knee joint load. (2) The combined effects of mental fatigue and decision making lead to increased knee extension moment and valgus moment, leading to characteristics that may increase the risk of injury. STUDY DESIGN: Descriptive laboratory study. METHODS: Twenty male amateur soccer players were recruited as participants. Mental fatigue was induced through a 45-minute Stroop task, where the color of the screen changed randomly using a self-made infrared induction synchronous trigger to create different anticipated state scenes. A 12-lens infrared spot 3-dimensional (3D) motion capture system and 3D force plates were used to collect kinematic and dynamic data from the trunk and knee joints. RESULTS: Mental fatigue and expected state demonstrated significant interaction effects on knee flexion angle and peak horizontal posterior GRF. At the initial contact, knee flexion angle ( CONCLUSION: Soccer players demonstrated a more upright landing posture post-mental fatigue, and unanticipated conditions negatively affected trunk and knee movement patterns, potentially increasing the risk of ACL injury. The interaction of mental fatigue with unexpected factors did not appear to amplify the risk of ACL injury. CLINICAL RELEVANCE: This study elucidates the effect of mental fatigue and decision making on the ACL. These risk factors may be incorporated into ACL prevention strategies.