Objective To analyze patterns of functional bundles in anterior and posterior cruciate ligaments during knee joint movement, so as to provide important references for studying injury mechanism of cruciate ligaments and implant reconstruction. Methods Five healthy knee cadavers were elaborately dissected to expose insertions of functional bundles in anterior and posterior ligaments on both the femur and tibia. CT scans and 3D finite element reconstruction with Mimics and ANSYS were conducted at 0°, 30°,60°, 90°, and 120° flexion angle of the knee joint. The center points of insertions and parallel sections of functional bundles defined by the software ANSYS and CATIA were connected to the centerlines, and at five different knee flexion angles, the lengths of centerlines, defined as the bundle lengths, were measured. Results The length of anterior medial bundle (AMB) of anterior cruciate ligament (ACL) increased gradually as the flexion angle grew from 0° to 90°, but slightly decreased at 120° flexion angle; whereas the length of posterior lateral bundle (PLB) of ACL decreased as flexion angle went from 0° to 90° and slightly increased at 120° flexion angle. In posterior cruciate ligament (PCL), both anterior lateral bundle (ALB) and posterior medial bundle (PMB) extended in length as the flexion angle went from 0° to 120°. The change of ACL and PCL bundle’s length was statistically significant (P<0.05). Conclusions ACL bundles functioned in a reciprocal manner and PCL bundles functioned in a complementary manner during knee flexion. Through establishing the finite element model of functional bundles in cruciate ligaments, the actual length of cruciate ligaments could be reflected, which provided a reasonable method for studying the changes of actual length of functional bundles in cruciate ligaments during knee flexion.