Parallel time domain simulation is one of the reliable and potential methods that might realize the real-time online transient stability analysis of power system. In the paper, a new parallel calculation method, based on waveform relaxation method, is proposed for power system transient stability analysis. In the method, the entire transmission network is first divided into several subsystems by the geographical location; for each subsystem, its neighboring systems are represented as equivalent voltage sources at the boundary buses; the tie line flows are considered as the communication waveform among the subsystems; then, time domain simulation for each subsystem is performed in parallel. The test result of the practical system indicates that the new parallel algorithm completely achieves the on-line real-time or even over-real-time calculation speed and could be applied to the on-line transient stability analysis of practical system.