A new style low loss and low polarization loss special asymmetric 1×5 optical power splitter is designed and optimized. The combination structure of gradually broadening waveguide and wide straight transition waveguide is introduced in the input waveguide during designing the Y-branch. The input optical field become gradually wide, the loss of Y-branch is obviously deceased. The power of first output port and second to fifth output port take 50% respectively. The three dimension beam propagation method is used to simulate and optimize the special asymmetric 1×5 optical power splitter, the simulation result indicate that the 1×5 splitter have the good uniformity, low insert loss, low polarization loss, and small dimension. Between the wavelength range 1 250~1 650 nm, the excess loss, the uniformity and polarization loss of first output port is less than 0.07 dB, 0.023 dB and 0.009 dB respectively. The uniformity and polarization loss of second to fifth output port is less than 0.45 dB, 0.41 dB and 0.06 dB respectively.