Abstract: An efficient method for computing signal propagation through cold plasma or waveguideis presened in this paper. This method deforms the Bromwich contour to seperate it into clos contoursaround the poles and the branch cuts. Integeration around the branch cut can be transformed into aFourier transform in a finite range, and the transformed function has good property, so only small numberof points are needed to obtain satifying results through IFFT. This method provides an effdent toolfor dispersion problems.