[Objective]To compare the biomechanical behavior of knee joints with mild and severe osteoarthritis (OA), and to elaborate the biomechanical mechanism of the progression of knee OA. [Methods] The left mild OA knee joint finite element model (KL grade I) and the right severe OA knee joint finite element model (KL grade IV) of the same patient were constructed respectively. Under the same setting of material properties, boundary conditions and loads, finite element analysis was carried out to compare the changes characteristics of knee meniscus, femoral cartilage and tibial cartilage between mild and severe OA, including contact area，contact pressure and Mises stress. [Results] The total contact area and lateral compartment of severe OA knee joint was larger than that of mild OA knee joint, but the contact area of medial compartment was smaller than that of mild OA knee joint. . The peak values of contact pressure, Mises stress on meniscus, femoral cartilage and tibial cartilage of knee joint with severe OA were higher than those of knee joint with mild OA; The contact pressure and peak Mises stress on the lateral meniscus of both knees are greater than that on the medial meniscus, and the contact pressure and peak Mises stress of the medial compartment between femoral cartilage and tibial cartilage of both knees are greater than that of the lateral compartment. [Conclusion] The stress distribution of knee joint with severe OA is different from that of knee joint with mild OA. The changes of contact area, contact pressure and peak Mises stress are related to factors such as meniscus dislocation and cartilage defect. The changes of biomechanical behavior and anatomical structure promote the progress of knee OA.