Objective To obtain biomechanical conditions of the tibia section with different osteotomy thickness under different activity states, so as to provide theoretical basis for osteotomy thickness in total knee arthroplasty (TKA) and activities of patients after operation. Methods The three-dimensional models of the lower limb were reconstructed and the proximal tibia was resected by 0 mm,5 mm,7 mm,9 mm respectively based on the principle of bone resection in TKA. The tibia model after osteotomy were assigned the material properties and the stress and strain of the tibia cross-section were analyzed under the following activity states: standing, walking, running and going upstairs. Results The stress and displacement of the tibia cross-section increased with the increase in osteotomy thickness under the same activity state. The maximum stress and displacement of tibia cross-section increased in general with the same osteotomy thickness, when the strength of activity increased. Conclusions During TKA in clinic, the stress and strain on tibia cross-section became larger as the tibia osteotomy became thicker and the postoperative activity became stronger. Excessive osteotomy in TKA and high intensive activity after TKA should be avoided, so as to reduce stress and strain of the tibia plateau and promote long-term life of the prosthesis.