The birefringence of one-dimension PMMA surface gratings on a gold film substrate is investigated. The grating served as a coupler to facilitate the incoming light coupled to surface plasmon wave (SPW) which possesses high propagation wave vector. Since surface plasmon waves(SPWs) have a special dispersion relation, the birefringence £Gneff (£Gneff =nx-ny, where grating k-vector is along x axis) of this structure is relatively large and can be changed from positive to negative by changing the operation wavelength. The obtained the four Stokes parameters at 515nm and 633nm are marked on the Poincare sphere. £Gneff is 2£k/7 and -£k/8 at 515nm and 633nm respectively. Further, by changing the form factor of PMMA gratings, we found that the maximum £Gneff occurred when PMMA stripe width : air gap=1:1.