A cascading failure parallel simulating software which can simulate working of many relay protections and automatic safety devices in power system is implemented based on MPI and user-defined modelling technique. First, by designing user-defined modeling approach using object-oriented method, modeling of many relay protections and automatic safety devices is completed. Then using parallel computing techniques of splitting power grid and grouping models, the large-scale power grid network is partitioned into several subnets assigned to these processes named subnet-process; and the grouped models for relay protections and automatic safety devices are assigned to these process named external process by group. The data after splitting grid and grouping models are submitted to the parallel cluster and the parallel simulating for cascading failure is starting. Because the subnet-process and external process are executed in parallel by different CPUs, simulating efficiency can be improved, and cascading failure simulating can be completed within an acceptable time of users.