The Kuosheng plant analyzer is a fast and accurate simulation program developed on the AD-100 peripheral processor systems. With the feature of the optimization module, the capability of the Kuosheng plant analyzer is highly improved. It is successfully applied to the feedwater control system (FWCS) design via the optimal approach eliminating the tedious trial-and-error tuning process. First, the level setpoint change transient performed during the startup tests was simulated to verify its accuracy. After verification, it was then applied to design the FWCS with the aid of the optimization module. The objective function is selected as the proper function of the design specifications. The optimization module adjusts the controller settings automatically by minimizing the objective function. The design process starts from the speed loop and continues to the level loop in a systematic way. All the design specifications are satisfied with the optimized controller settings and the control system performance is improved.