Objective To determine the effect of fatigue on the risk of anterior cruciate ligament (ACL) injuries using a controlled experiment.Method Thirty-two young male adults were selected.Half of them were asked to complete landing and sudden cutting tasks when feeling fatigue,while the rest were asked to conduct the same tasks without fatigue.Results The results indicated that fatigue led to increased knee flexion and knee rotation of the support leg during the initial landing,It also resulted in significant increase in the peak knee flexion,knee internal rotation angle and knee internal rotation moment during the double support phase.Moreover,the peak value of the knee flexion angle and knee internal rotation angle increased significantly due to fatigue during the single leg support period.Conclusion Being fatigue,people might take the adaptive posture control to decrease the ground shock,which in turn decreases ACL injury risks.However,it can be neutralized by the increase in the rotational angle.