Simulation for testing environment and equipment is very helpful for research， development and testing of power systems. The new characteristics of power plants and substation automation systems reported in recent years， such as the network-based， layered distributing， opening structure， and the development tendency of digital substations make it possible to develop a general-purpose testing equipment. Such a kind of simulation system， named the SimRTU system， is described with the design and implementation discussed in detail. The main merits of the SimRTU system are general applicability， portability， easy upgrading and clip ability.