One of the traditional methods of preparing Cu (In, Ga) Se2 (CIGS) is the three-step co-evaporation. Using this technique, four elements of Cu, In, Ga, and Se form a parabolic Ga gradient distribution through their inter-diffusion and interaction. In this paper, CIGS films and solar cells with different Ga gradient distributions are prepared through adjusting the temperature of Ga source. The effects of different Ga gradients on structural and electrical properties of CIGS film surface and back surface are studied through a variety of measurement methods. The influences of surface conduction band offset value and back surface electric field on the performance of solar cell are investigated and analysed. Finally, an optimal Ga gradient distribution is obtained, where the spectral response is improved, and the solar cell shows better performance.