This paper introduces a CFD-aided optimal design method for duct nets. Based on the structured frame, this method can help to improve the dynamic performance in the inner flow field of hydraulic manifold blocks (HMB). With the grid from geometry model of structured duct nets and the initial and boundary conditions from testing, the transient inner flow field of HMB duct nets is simulated. Basic flow characteristics in duct nets are obtained. Important factors involved are discussed and the procedures are illustrated.