With the increasing unit capacity in power plant, 6 kV auxiliary power system is more and more important for the stable and safe operation of units, especially its quick switching device. Now the new generating units commonly have used quick switching device. In this situation, a number of old power plants also begin to transform auxiliary power system from automatic input device to quick switching device. Taking the MFC-2000 quick switching device of No.5 unit in Douhe Power Plant as an example, the transformation process of quick switching device in 6 kV auxiliary power system and its actual effect after transformation are discussed, which could provide valuable experience for the following work.