Parallel technology is a powerful tool to provide the necessary computing power and memory resources for the Finite Difference Time Domain(FDTD) method to simulate electrically-large and complex structures. In this paper, a high performance parallel FDTD is developed for multi-core cluster systems. It employs Windows Socket(WinSock) to achieve efficient inter-process communication as well as multi-threading to make full use of the hardware resources of multi-core processors on a PC-cluster. Key steps for parallel FDTD such as synchronization, data exchange, load balancing, etc., are investigated, finally resulting in the development of a FDTD parallelization strategy. An experiment is presented with its results demonstrating the proposed Winsock and multi-threading-based parallel FDTD achieving speedup of 16.0 and efficiency of 53.3% when 10 processors with 30 cores are employed. It outperforms traditional parallel FDTD which uses either MPI or MPI-OpenMP，gaining speedup of 13.7,12.2 and efficiency of 45.8%,40.7% respectively under the same circumstance.