A hybrid model for describing a fully developed two-dimensional sea surface is presented, which incorporates the sea spectrum into a fractal model, and an improved two-scale model is applied to calculate the scattering coeffcient. In this model the slope distribution of the large-scale surface is obtained with a numerical method instead of being assumed, and the projecting and ray tracing methods are applied to obtain the shadow function fdr arbitrary incident and scattering angles. Moreover the effect of curvature of the large-scale undulating surface on the scattered wave is taken into account. Hence the precision of the results is greatly increased at low-grazing angles. Finally the amplitude and fractal characteristics of the scattered wave are analysed, which are of great significance for distinguishing and detecting targets on the sea.