The sulfur-transfer catalyst plays an important role in the fluid catalytic cracking (FCC) process through which the sulfur oxides (SOx) are removed from flue gas. This work reports the synergistic effect between CeO2 and MgO on the SOx oxidation and adsorption in the FCC process. The CeO2 acts as the oxidation promoter, while MgO is the active phase for SOx adsorption. The influence of crystal size on the synergistic effect between CeO2 and MgO on the SOx oxidation and adsorption was investigated. It was observed that the initial apparent SOx pick-up rate decreases with the increase in the CeO2 crystal size, while the maximum SOx oxidation and adsorption capacity decreases with the increase in the MgO crystal size. Therefore, this work provides a detailed understanding of the synergistic effect between CeO2 and MgO for the SOx pick-up, which is crucial for the rational design and preparation of the sulfur-transfer catalysts.