The fabrication tolerance of a short and compact low refractive index grating waveguide polarisation splitter based on the principle of resonant tunnelling was analyzed in this study. The design utilised two grating waveguides with an intermediate conventional waveguide layer. The design and optimisation were conducted using the quasi 2-D effective index solver with global search algorithm. An optimum device operating at 1.55 μm wavelength was obtained at a length of 340 μm. The splitting ratios were calculated to be 36 dB and 15 dB, and the overall device transmission efficiencies, after considering the three-dimensional waveguide leakage loss, were estimated at 88% and 83% for tranverse magnetic and tranverse electric polarisation, respectively.