We describe a compact double-layer waveguide grating splitter that not only achieves efficient coupling between single mode fiber and a silicon-on-insulator optical waveguide but also realizes effective splitting. By appropriate choice of waveguide/grating parameters, including thicknesses, periods, height, and fill factor to optimize the mode matching, coupling efficiency is improved and the value of power difference of each output port is also significantly decreased. The maximum of power difference between four output ports is about 6.2%; however, the minimum value is only 0.6% or so. Moreover, the average power difference of four output ports is lower than 10% for TE polarization light over the $10\mathrm{nm}$ wavelength bandwidth centered at $1.54\mathrm{\mu m}$ . In addition, the splitter structure has the best tolerance for grating fabrication with deviations of grating depth $90\mathrm{nm}$ .