This paper mainly covers the issue of Finite-Difference Beam Propagation Method (FD-BPM), which is one of the numerical methods of optical waveguide calculation. In order to discuss the problem and bring in the foundation of the method, the following problem is discussed ordinarily: theory of optical waveguide, numerical method of optical waveguide, difference method of partial differential equation. With the basis, the FD-BPM becomes clear. After all these theories, the boundary condition, especially the transparent boundary condition (TBC) are discussed. At last, a concrete optical waveguide structure is inducted. A series of calculations and discussions are followed in order to compare the usage of different parameters.