Abstract: Regarding the robustness and dynamic response issue of unstable second order process with time delay, we propose a twice optimal control structure adjustment method to realize its optimization control. First, we use twice optimal control principle to design infinite dimensional controller and observer for the system. Then we increase the local feedback K_f value of structure adjustment to eliminate the oscillation during the transition process of twice optimal control system. Finally, simulation studies have been demonstrated to show the tracking response and disturbance resistance robustness advantages of the proposed method compared with two other schemes including PID based on internal model control (IMC) filter and modified Smith.