Servo press has the ability to improve the formability, quality, and productivity of metal products by working with various forming curves of slide. However, the operations of free-motion function are not very intuitional and efficient for most of servo presses so that it is difficult for operator to construct self-design forming curves. This article proposed a control system design of servo mechanical press which consists of an open-system controller and a real-time operating system to provide a convenient and efficient approach for operating free-motion function. In the proposed control system, operators determine the motion of slide by adjusting several control points of forming curve in a friendly and graphical interface. Then a calculating process based on the B-spline and curve fitting theories is applied in the controller to transform settings of operator into speed commands of servo motor automatically. Through this process, the controller is capable of driving servo motor smoothly and generating an expectant forming curve. In the latter section, some compare and analyses of setting of operator and actual results are demonstrated to verify the performance of proposed control system. By applying the proposed control system design to servo mechanical press, operators are able to create practical and satisfying forming curves more easily.