Objective To quantify biomechanical characteristics of knee joints for athletes after anterior cruciate ligament reconstruction (ACLR) surgery during side cutting under unanticipated conditions, and to investigate the potential risk of anterior cruciate ligament (ACL) injury in athletes. Methods The infrared motion capture system and three-dimensional (3D) force platform were used to collect the kinematic and kinetic data of lower limbs of 30 male athletes with ACLR during side cutting under anticipated and unanticipated conditions. The angle, moment, ground reaction force (GRF) and other test parameters 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 healthy limb. The internal rotation moment and tibial anterior shear force of the affected limb were significantly higher than those of the healthy limb. Compared with the anticipated conditions, the knee flexion angle, internal rotation moment and tibial anterior shear force significantly increased under unanticipated conditions. The knee external rotation angle of the affected limb was significantly larger than that of the healthy limb under both anticipated and unanticipated conditions. Under unanticipated conditions, the lateral GRF and anterior-posterior GRF of the unaffected limb were significantly lower than those under anticipated conditions, and the lateral GRF of the affected limb was significantly higher than that under anticipated conditions. Conclusions Compared with the healthy limb, the affected limb of athletes with ACLR showed biomechanical characteristics of smaller knee flexion angle, larger knee internal rotation moment and anterior tibial shear force during side cutting, indicating that the affected limb might have a greate potential risk of ACL injury. Under unanticipated conditions, both lower limbs of athletes with ACLR showed biomechanical characteristics of increased knee internal rotation moment, lateral GRF and anterior tibial shear force, indicating that the potential risk of ACL injury is not limited to the affected limb, and ACL injury prevention should also be strengthened in healthy lower limb during side cutting.