Surface plasmons（SPs） generated in nano metallic gratings on medium layer can greatly enhance the transmission field through the metallic gratings. The enhancement effect is achieved from λ=500 nm to near-infrared domain. The enhancement rate is about 110% at the wavelength of about 610 nm and about 180% at λ=700　nm and 740 nm where most kinds of thin film solar cells have a high spectral response. These structures should provide a promising way to increase the coupling efficiency of thin film solar cells and optical detectors of different wavelength response.