A statistical optimum design method suitable for wide waveband wavelength insensitive coupler (WINC) is proposed. Using the method, silica waveguides type 50% and 20% coupling ratio WINCs are designed with the optimization of the polarized light fluctuation. The design results are verified by the 3D BPM simulation, and show that the coupling ratios of (50±2.1)% (the polarized fluctuation <0.65%) and (20±1.4)% (the polarized fluctuation <0.20%) in a wide wavelength region of 1280 nm～1650 nm are realized, respectively.