This paper shows that optimal preview control system approaches to a decoupled preview control system as the weighting factors for the control input variables in the quadratic performance index tend towards zero. Therefore, to examine the characteristics of the decoupled preview control system also means to examine the asymptotic characteristics of the optimal preview control system. By this property, relationships between the design parameters and the response of the optimal preview control system become clear and the design of the optimal preview control can be simplified by utilizing the properties that the decoupled preview control system features. Further, the decoupled preview control method is applied to the vector control system for an induction motor drive. By this application, a trade off between the conditions of the vector control and the magnitude of input variables becomes easy. To demonstrate the significance of this proposed method, simulation results of this application are carried out.