Abstract: Supported by AutoCAD, Visual Basic for Applications and Drawing Exchange Files are used to develop a software of auto dissection with three-dimensional grid for the finite-difference time-domain electromagnetic simulation. Firstly, simulation structures are plotted in AutoCAD drawing district. Then these structures are dissected by a program of Visual Basic for Applications. And these masseges are saved as DXF format. Locations and electromagnetic parameters of each grid are extracted by a Fortran program. Finally, these data are sent to a finite-difference time-domain program for electromagnetic simulation. Correctness and practicability of the software are validated through an example of microstrip feed metal patch.