This paper presents an approach to refine H∞-optimal controller for the 4-block H∞ control problem. The diagonalizing matrix pair which connects the successive layers is constructed. The second singular value of the compensated system can be analyzed and synthesized with a free parameter by the proposed approach which is implementable in computation with appropriate selection of diagonalizing matrix pair. The H∞-norm of the sub-layers can, therefore, be improved. Furthermore, an engineering application concerning the rotorcraft flight control is provided. The simulation results bestow a promising progress in frequency domain.