Background: During the robot-assisted pelvic fracture reduction process, the clamping instrument are subjected to the large reduction force from the robot, resulting in inevitable great stress concentration and deformation of the bone pins, affecting the fracture reduction accuracy. Method:A compact and easily-to-adjust clamping instrument with active variable stiffness is designed. The relationship between the component's elongation and the clamping instrument's deformation and stiffness is derived and calculated.Furthermore, the finite element model of the fixed and the variable stiffness clamping instruments connecting with the injured pelvic musculoskeletal tissue is developed, respectively.Results: Applying the same reduction force, the deformation of the clamping instrument with variable stiffness is reduced, especially in the elongated state.Moreover, the new clamping instrument meets the strength requirements and makes a better stress distribution. Conclusion:The clamping instrument can achieve stiffness adjustment during the reduction process and will be used for improving the surgery accuracy.