Retrieving the oil-water mixture ratio (M, the proportion of oil in an oil-water mixture) of the marine oil spill is of vital importance for the emergency treatment of oil pollution, which can be achieved by a copolarization ratio (PR) method. In the framework of the PR method, it is assumed that Bragg scattering dominates at moderate incidence angles, and the relative contribution of non-Bragg scattering to total scattering (RCNT) is typically neglected. However, in this work, it is found that the RCNT cannot be neglected for L-band, especially for HH polarization at higher wind speeds (the RCNT is approximately 0.6-0.8 for wind speeds of 10-12 m/s). The significant impact of non-Bragg scattering results in the PR method not being accurate enough for retrieving M. After separately investigating the influences of the damping effect and effective dielectric constant reduction on the RCNT, a novel approach is proposed for more accurately estimating the M of marine oil spills by eliminating the impact of non-Bragg scattering. The proposed method was applied to uninhabited aerial vehicle synthetic aperture radar (UAVSAR) images collected during the Deepwater Horizon (DWH) oil spill accident. The inversion results of our approach show that the M of most oil spill areas ranges from approximately 0.3 to 0.6, which matches the reality better than that of a traditional PR method (ranging from approximately 0.6 to 0.8).