Abstract: A parameter optimization method based on stiffness sensitivity analysis was proposed for the structural optimization of slender manipulator. Taking a 3-DOF inner-wall grinding robot of the solid-propellant rocket engines as an example, both the structural model and the stiffness model are established for the manipulator. Sensitivity analysis of the effect of the wall thickness on the bend stiffness is then performed for seven major parts. According to the results, four thickness variables which are sensitive to the stiffness are found out. With the four variables as the design variables, final optimization scheme is obtained after optimizing the manipulator structural parameters. The results show that manipulator stiffness is improved 7.04% after parameter optimization. This work lays a theoretical foundation for the structural optimization for such manipulators.