The traditional numerical method of calculating electromagnetic scattering from the dielectric sea surface requires large amounts of memory and computation time as irradiated area increasing rapidly at low grazing angles. The method of Sparse Matrix Canonical Grid (SMCG) computes the product of the Taylor expanded flat surface matrix and the surface current column vector in far field by the Fast Fourier Transform (FFT), which decreases the computation complexity efficiently. According to the properties of the Green’s functions of lossy dielectric and free space, the Physics-Based Two-Grid (PBTG) calculates surface field solutions on the both of dense and coarse grids, which reduces the amounts of memory required. Predictions of an exact numerical model using SMCG-PBTG based on Monte Carlo simulation are compared with experimental data. Experimental data is obtained from wave tank experiments in which the backscattering patterns of 1D sea surfaces with PM spectrum at S- and Ku-band are measured. The sea surfaces corresponding to low and moderate windspeed can be directly simulated in wave tank, and the scale model provides an alternative approach for measuring scattering from sea surfaces corresponding to high windspeed. A comparison of the absolute value of the backscattering coefficient shows the theory and experiment to be in good agreement. Results show that the correlation lengths and scattering behaviors are significantly different under the different windspeed.