Abstract: The stiffness is a crucial problem for a space manipulator.It is very difficult to construct a high stiffness space manipulator with limited mass since high stiffness manipulator usually requires high expenditure on mass and volume.A method of stiffness optimization was presented to improve the stiffness of a space manipulator with limited mass.First,the stiffness model was established.Then,an optimization algorithm based on mass allocation was proposed.Finally,the optimum proportion between component mass and dimension was calculated.