To improve the simulation efficiency of a UHVDC system, a multi-core parallel simulation method based on PSCAD/EMTDC is proposed. Through model splitting and multi process parallel operation, this method solves the problem of inability to run or low efficiency in the offline electromagnetic transient simulation of a large-scale DC system. First, high-precision modeling methods and principles of primary systems and control and protection devices are introduced. Then, for the primary system, based on the natural decoupling of long lines, an equivalent short line decoupling method based on the existing components of the main circuit is proposed. This solves the problem of parallel interfaces in the case of no line or short lines, and the decoupling and network division scheme of the primary system is formulated. Finally, in order to solve the problem of communication and synchronization between multi simulation processes, a multi process parallel method based on shared memory is designed. Taking the Yanmenguan-Huaian UHVDC project as an example, a multi-core parallel model is built. Through comparative experiments, it is verified that the parallel method can greatly improve simulation efficiency without affecting accuracy. This work is supported by the Science and Technology Project of the Headquarters of State Grid Corporation of China “Research and Application of Protection and Regulation Auxiliary Check Technology based on Multi-Scenario Simulation of UHVDC Converter Station” (No. SGJ0000DKJS1900863).