In order to design and make multimode interference (MMI) couplers, the imaging positions must be located accurately. The implicit finite difference beam propagation method (BPM) in three-dimensional alternate directions is used for the analysis of strongly guided and weakly guided multimode interference couplers. The length of multimode waveguide, the positions of input guides and output guides are determined precisely. The equivalent width Weq and the beat length Lc of two lowest modes of multimode waveguide are obtained first by the numeric modeling of a symmetric interference MMI, and then these parameters together with BPM are used for the device design. The imaging positions showed the close results between BPM and the predictions of the mode propagation analysis (MPA) theory,however, Weq and Lc were obtained by the BPM, MPA theory will not give any result without these parameters. The three dimensional (3D) waveguides are not treated as 2D waveguides by means of effective index method (EI), therefore, this method can be used directly to 3D waveguides. As a kind of numerical method, some normal approximations are not introduced as MPA theory, so it will ensure the precision of calculation and can afford reference for practical MMI device design.