We develop the prototype control system of XFEL/SPring-8　undulator and confirm the capability of the system. The XFEL consists of the 8GeV linear accelerator and the undulators. The undulator is important for lasing of the intense X-ray beam. For the lasing, the required positioning accuracy of the undulator gap is less than 1 micrometer. We have to control the undulator gap to be less than sub-micrometer resolution. To realize, we control the undulators by the periodic feedback with the linear encoders. In this paper, we will describe detail design of the undulator control system on XFEL.