In recent years, the scale and complexity of Chinese power grids have increased continuously, making it increasingly difficult for existing offline setting methods to meet the requirements of relay protection. With the rapid development of wide-area information measurement and power grid communication technologies, relay protection online setting undoubtedly is an effective measure to solve the above problems. However, the fast short-circuit calculation of the large-scale power system network structure and operation mode is an important prerequisite for online setting technology development. For this reason, this paper proposes a power system disturbance area division method. Based on this, the large-scale power grid is decomposed into several small sub-networks to simplify the short-circuit calculation process. Furthermore, a new parallel computing method of fast short-circuit calculation based on LDU decomposition method is proposed. The calculation results of the program based on the OPENMP parallel computing platform proved that the computing speed of the new method is significantly improved compared with the traditional single-core serial computing. The new method provides a new idea and solution for the development of online setting technology. This work is supported by National Natural Science Foundation of China (No. 51677108) and Science and Technology Project of State Grid Corporation of China (No. SGTYHT/16-JS-198).