Objective To compare the biomechanical behavior of knee joints with mild and severe knee osteoarthritis (KOA), and to elaborate the biomechanical mechanism of KOA progression. Methods The finite element models of the left mild KOA knee (KL grade I) and the right severe KOA knee joint (KL grade IV) for the same patient were constructed respectively. Under the same setting of material properties, boundary conditions and loads, the change characteristics of knee meniscus, femoral cartilage and tibial cartilage between mild and severe KOA, including contact area, contact pressure and von Mises stress were compared by using finite element analysis. Results The total contact area and lateral compartment of knee joint with severe KOA was larger than that of knee joint with mild KOA, but the contact area of medial compartment was smaller than that of knee joint with mild KOA. The peak contact pressure, von Mises stress on meniscus, femoral cartilage and tibial cartilage of knee joint with severe KOA were higher than those of knee joint with mild KOA. The contact pressure and peak von Mises stress on the lateral meniscus of both knees were greater than those on the medial meniscus, and the contact pressure and peak von Mises stress of the medial compartment between femoral cartilage and tibial cartilage of both knees were greater than those of the lateral compartment. Conclusions The stress distribution of knee joint with severe KOA is different from that of knee joint with mild KOA. The changes of contact area, contact pressure and peak von 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 KOA.