Objective To explore the biomechanical properties of the anterior cruciate ligament (ACL)fracture knee joint at different knee flexion angles,we reconstructed the ACL fracture and normal three-dimensional solid model of the knee joint based on CT and MRI images of human’s knee joints. Methods Combined with the Unicompartmental Knee Arthroplasty (UKA),we implant a Oxford III prosthesis and analyze them in finite element methods. We established the 3D finite element model for UKA at 0°, 30°, 60°, 90°, and 120° of knee flexion. The tibial prosthesis is inclined backward by 7°. 1000N compressive load is applied on the center of femur to analyze the stress distribution of femoral, meniscus, and tibial prosthesis under different knee flexion angles.Results The research of knee ACL fracture group and normal group in five knee flexion indicates that the Von Mises stress of the meniscus liner was statistically different between the ACL rupture group and the normal group. The maximum difference was 62.5%. At 0°,30°, 60°and 120° of knee flexion, the maximal stress in the ACL rupture group was significantly higher than that in the normal group; At 30° of knee flexion, the maximum stress the femoral prosthesis in the ACL rupture group increased significantly. The maximum increase in other four knee flexion states is 60.81%; Compared with the normal group in the five knee flexion states, the maximal stress of tibial prosthesis in the ACL fracture group is respectively increased by 19.07%、36.78%、25.69%、-4.38%、51.19%. In the ACL rupture group, the uniform stress on the meniscus liner and the tibial prosthesis are higher than that in the normal group except at 90° of knee flexion. Greater damage can be caused to the meniscus liner at 30° and 120° of knee flexion. Conclusions The results of the knee joint ACL fracture in UKA can provide a theoretical basis for the surgical design and the optimization of the meniscus prosthesis.