After preparing the large cross section single mode rib waveguide and it′s Y power splitter of GeSi alloy, this paper studies (1) FD BPM is used to simulate the light propagating in the splitter and the comparison between FFT BPM and FD BPM. (2) The simulation of λ =1.3 μm light propagating in the designed 1×4 splitter. Results show that compating to FFT BPM, FD BPM is easier to deal mathmatiocally and computing speed is faster and not very sensitive to the change of Δz . The simulation of 1×4 power splitter is succeddful. When the single-mode Gauss beam incidents into the splitter, the light propagated in the device according to requirement. At mid-way it splites into 2-beam and inot 4-beam finally.