Power grids reliability and security are influenced by transient behaviors of various power system components. However, analyzing these transient behaviors needs to estimate millions of time-domain state variables with high computational efficiency. Most of the existing simulators are not based on parallel software platforms which have been proven to be powerful in many other applications. They are thus incapable of simulating transient behaviors of large-scale power systems efficiently. The lack of powerful dynamic simulation tool has become a major bottleneck towards the smarterization of power grids. Inventing a software based novel fully parallel topology of power systems, we propose a multi-Threading based parallel simulator for transient behavior analysis in this paper. The accuracy and efficiency of the proposed simulator has been evaluated on IEEE and Polish standard bus systems, against the state-of-The-Art open source software package (MatDyn). The results show that the runtime of the proposed simulator attains up to 80.49 ×speedup compared with MatDyn on 12-Thread CPUs.