A concurrent subspace optimum design method for composite stiffened panel is proposed. It divides the entire optimization into three sub-optimixations, that is layout optimization, size optimization and ply optimization, and the variables of them are the form of the stiffeners, the size of stiffeners and the ply thickness. All the sub-optimizations are solved independently. Once the three sub-optimizations finished, the values of the variables are reset according to the optimization results, and then to repeat all of the above until the convergence requirement is meet. The optimization design of three composite stiffened panels is conducted to demonstrate the effectiveness.