The grating structure on metal contact electrode is designed to couple incident light into surface plasmon ploaritons, and a calculated approach is presented to quantitatively evaluate light trapping properties. It can be found that cube grating made by copper material can obtain the largest efficiency of thin film solar cell. The effect of the structure parameters of solar cell on the light trapping is discussed. An increasement in light trapping will be maximum with 30 nm height grating. The length of grating determines the width of peak in the light trapping spectra, and the period of grating determines the location of peak in the light trapping spectra. The results obtained have significance in further design to get more efficiency thin-film solar cells.