This paper proposes a new simulation technique for digital spare power automatic switching. The technique analyzes the logic of digital spare power automatic switching based on power system dynamic topology. The simulation software based on Sophic platform is developed using the proposed technique. The developed software is deployed in a distributed fashion and is used in the simulation environment which do not affect the normal functions of SCADA. The simulation technique has been applied in Hong Kong Chui Ling Road station, which achieves the expectation.