Abstract: In order to figure out the distortion of radiation pattern, which is caused by backward leakage, second contamination and output boundary truncation, a novel Local Waveguide Port (LWP) technique based on FDTD technique is proposed in this paper. And the detail of boundary conditions and boundary parameters are given for the method. At the end of this paper, this LWP technique is utilized to analyze discontinuous rectangular waveguide, micro-strip antenna and slot antenna array excited by local waveguide port. The calculation results show that the high accuracy can be obtained in the discontinuous problem and the much better result can be achieved in the radiant problem. Consequently, this method can be used in both discontinuous problem and radiant problem.