In the sequential sputtering/selenization process, Ga segregation at the back of Cu(In, Ga)Se2 (CIGS) absorber is frequently observed. In this paper, Ga diffusion in CIGS absorber is investigated during the sputtering and selenization process. Results show that Ga diffusion is closely related to Cu/(In+Ga) ratio in the metallic precursors and the selenization temperature, but barely influenced by Ga/(In+Ga) ratio in the metallic precursors. Based on Fick's second law, a simplified model is established to describe Ga diffusion from the back to the surface of CIGS absorber, which suggests that Ga diffusion coefficient is the dominant factor to constrain Ga content near the absorber surface. By process optimization, Ga/(In+Ga) ratio near the absorber surface is successfully increased. Accordingly, a CIGS solar cell device with efficiency of 12.42% has been obtained.