In this study, we add an additional layer above and under the CIGS absorber layer as a secondary absorption layer respectively. We made the conventional structure of ZnO/CdS/CIGS/Mo becomes the structure of ZnO/CdS/CIGS/InGaP/Mo and ZnO/CdS/InGaP/CIGS/Mo which can improve the conversion efficiency. And we translate the thickness proportion of Ga and the doping concentration to find out the best parameter. According to the simulation, the wavelength of EQE in 600 nm ~ 1200 nm for our proposed CIGS solar cell which the additional layer under CIGS layer has been improved when compared to the conventional CIGS solar cell. The short-circuit current density has been increased about 9 %. And the conversion efficiency has also been increased about 9 %.When the additional layer above the CIGS absorber layer, according to the simulation, the wavelength of EQE in 300 nm ~ 600 nm for our proposed CIGS solar cell is improved when compared with the conventional CIGS solar cell. The short-circuit current density has been improved about 7.7 %, the open-circuit voltage about 7.1 %, and the conversion efficiency about 20.6 %. The main reason is that when the InGaP absorption layer under the CIGS layer which can catch the light which can¡¦t be absorbed by CIGS layer. The InGaP absorption layer above the CIGS layer which can catch the light immediately.