A new primary system model of simulation substation suitable for parallel computing is proposed in order to decrease equipment cost and enhance simulation quality of simulation substation. Several measures are adopted such as : reduce nodes of simulation model by short circuit fault transfer， separate line model from substation equipment model by using Bergeron's circuit， split the substation equipment model by asynchronous substitute and give selection principle of overlap circuit. Prototype tests show that the simulation model built on Bergeron's circuit and asynchronous substitute suits for parallel computing， and the digital primary system may operate well with the real secondary devices.