Objective: This study was to quantify the biomechanics of knee during side cutting of athletes after Anterior Cruciate Ligament Reconstruction (ACLR) under unanticipated conditions, and to investigate the potential risk of Anterior Cruciate Ligament (ACL) injury in athletes. Methods: The infrared motion capture system and 3D force platform were used to collect the kinematic and kinetic data of the lower limbs of 30 male athletes after ACLR during side cutting under anticipated and unanticipated conditions. The angle, moment, ground reaction force (GRF) and other test indexes were analyzed statistically by using two-way ANOVA with repeated measures. Results: The knee flexion angle, valgus moment and flexion moment of the affected limb were significantly lower than those of the unaffected limb. The internal rotation moment and tibial anterior shear force of the affected limb were significantly higher than those of the unaffected limb. The knee flexion angle, internal rotation moment and tibial anterior shear force significantly increased under unanticipated conditions compared with the anticipated conditions. The knee external rotation angle, lateral GRF, and anterior and posterior GRF during side cutting had significant interaction effect. The lateral rotation angle of the affected limb was significantly higher than that of the unaffected side under both anticipated and unanticipated conditions. Under unanticipated conditions, the lateral GRF and anterior and posterior GRF of the unaffected limb were significantly lower than the anticipated conditions, and the lateral GRF of the affected limb was significantly higher than the anticipated conditions. Conclusion: (1) Compared with the unaffected side, the affected side of ACLR athletes showed the biomechanical characteristics of smaller knee flexion angle, larger knee internal rotation moment and anterior tibial shear force during side cutting, which may suggest that the affected side have greater potential risk of ACL injury; (2) Under unanticipated conditions, both lower limbs of ACLR athletes showed the biomechanical characteristics of increased knee internal rotation moment, lateral GRF and anterior tibial shear force, which suggest that the potential risk of ACL injury is not limited to the affected side, and ACL injury prevention should also be strengthened in the unaffected lower limb during side cutting.