Abstract: The Ag coating with thickness of 400 nm was electrochemical deposited on the SiO₂ matrix, and then the composition was characterized by scanning electron microscope energy spectrometer, the results were abnormal. The causes of abnormal phenomenon was analyzed by atomic force microscope (AFM) and Monte Carlo Simulation software. The results show that the characteristic X-ray energy of element O is K_α=0.525 keV, the low energy level was difficult to penetrate the Ag coating with thickness of 400 nm. The characteristic X-ray energy of element Si is K_α=1.74 keV, K_β=1.838 keV, the higher energy level can penetrate the thickness of Ag coating.