Solid oxide fuel cells (SOFCs) have been attracting people's attention for their high energy conversion efficiency, good fuel compatibility, no precious metal catalysts, and pollution-free emissions. However, the high operating temperature (800-1200℃) of the traditional SOFC can reduce the long-term stability and cause the difficulties in either the selecting of material or the sealing of SOFC. Therefore, great efforts have been devoted to developing the intermediate temperature SOFC (IT-SOFC), which works at 600-800℃. In the IT-SOFC, the ionic conductivity of doped CeO2-based electrolyte has a significant advantage relative to that of the conventional yttria-stabilized zirconia (YSZ) electrolyte. For example, at 600℃, the ionic conductivity of Sm-doped CeO2 is 0.02 S/cm much higher than that of the traditional YSZ electrolyte (only 0.0032 S/cm). Therefore, the doped CeO2-based electrolyte is a very promising electrolyte for IT-SOFC.Recently, the co-doping of two different elements into CeO2 has become a hot research topic. The ionic conductivity of Sm3+ and Sr2+ co-doped CeO2 has proved to be nearly twice as high as that of Sm3+ doped CeO2 (SDC). However, the mechanism for the co-doping effect on the conductivity of CeO2 is not clear. In this paper, Sm3+ and Sr2+ co-doped CeO2 is systematically studied using the DFT+U method. The microscopic properties of the Sm3+ and Sr2+ co-doped CeO2 including electronic density of states, band structure, oxygen vacancy formation energy and oxygen vacancy migration energy and so on have been calculated and analyzed by comparing with those of the Sm3+ or Sr2+ single doped CeO2. The calculation results indicate that Sm3+ and Sr2+ co-doping has a synergistic effect on the performance improvement of CeO2-based electrolyte, which can not only suppress the electronic conductivity of doped CeO2 system, but also can reduce the oxygen vacancy formation energy on the basis of single doped CeO2. The existence of Sm3+ can help to reduce the trapping effect of Sr2+ on oxygen vacancies, meanwhile the addition of Sr2+ can further reduce the minimum oxygen vacancy migration energy on the basis of SDC. Calculations by the climbing image nudged elastic band (CINEB) method indicate that the oxygen vacancy migration energy of the co-doped system can reach as low as 0.314/0.295 eV, which is lower than the minimum oxygen vacancy migration energy of SDC. Our research reveals the synergistic mechanism for Sm3+ and Sr2+ co-doping effect on the conductivity of CeO2, which is of great instructive significance for the further research and development of other high-performance co-doped electrolyte materials in IT-SOFC.