In this thesis, the major discussion is to improve the computer calculation time with the original simulation program in 3D rectangular coordinates. The small-angle method is used to divide the original silicon component into two parts. One is silicon region and the other is silicon-oxide region. The silicon region is inside the small-angle region, and we use the added silicon-oxide region to meet the simulation in 3D rectangular coordinates. Using this method, the simulation time is reduced to only one percent of the original. Further, the developed method is verified for its validity by the test with different parameters. So the small-angle method in 3D rectangular coordinates is workable and valuable.