A triplexer was fabricated based on asymmetric Y branch waveguides Cascade Multimode interference(MMI) for fiber to the home(FTTH). The simulation results of spectral response show that output optical fields are clearly. And the demultiplexing function for download 1 490 nm and 1 550 nm and multiplexing function for upload 1 310 nm were achieved. The simulation results of finited different beam propagation method (FD-BPM) show that insertion loss is less than 1.49 dB; the bandwidths of three response wavelengths meet the demand of ITU. 984 completely; the isolation for 1 310 nm is more than 47 dB; the isolation for between 1 490 nm and 1 550 nm is more than 29 dB.