Basing on a directional coupler we study the differences in simulation accuracy between 2 D BPM and 3 D BPM and the effect of calculating parameters size and initial effective refractive index.on simulation accuracy that include the number of the grid points,propagation step.During simulating,the method of effective refractive index is employed to transform 3 D waveguide to 2 D waveguide.The results show that the 2 D BPM agrees with 3 D BPM.Our study indicates that 2 D BPM is also accurate and propagation step size or initial effective refractive index doesn't affect the simulation results evidently,but the number of the grid points can lead to great influence.