A thin film solar cell with a grating in the layer was designed to increase the absorption of gallium arsenide thin-film layer.By using rigorous coupled wave method,the absorption efficiency in the range of 280~900 nm was analyzed for GaAs layer with single and double rectangular gratings.When the thickness of absorption layer obtained under certain circumstances,the rate of absorption compared to the rectangular grating structure of the non patterned absorber layer was greatly improved.The absorption rate of double rectangular grating increased by at least 10% over single rectangular grating and 20% over non patterned,which is more obvious.The results indicate that,by designing the structure,random reflection photonic in grating increases the effect of time and distance in the light absorption layer,enhanced the absorption rate and finally improve the photoelectric conversion efficiency of the thin film solar cell.