Polarization dependence of sum frequency generation (SFG) spectroscopy has been widely discussed to detect molecular orientation at surfaces. The present work examines the orientational analysis by molecular dynamics (MD) simulation of methanol/water mixture surfaces with varying concentrations. We calculated by MD the surface structure of the solutions and their SFG spectra in the methyl C-H stretching region, and directly analyzed the relations. The MD calculations reported that (i) the SFG signal of the methyl symmetric stretching exhibits a turnover behavior with increasing concentration and (ii) the polarization ratio is almost invariant over the concentration, while (iii) the orientation of the methyl group significantly randomizes in high concentration. The present work elucidated these three findings in a consistent manner by analyzing the MD calculations.