Abstract: We present a new nonlinearized design method for a second-order dead-time system with optimal control. First, on the basis of the twice-optimal control theory of the second-order dead-time system, we construct a system infinite-dimensional state observer and verify that the observer can eliminate the impact of time delay on the system and has strong robustness. Second, to improve the rapidity of the system, we apply a square-root amplifier to nonlinearize the system. Finally, we illustrate a nonlinear example and a comparative study. Results show that the new system has not only a fast and smooth tracking response but also strong robustness to load the disturbance and the parameter perturbation. The new method has strong applicability and is easy to implement.