The research into the co-segregation behavior of Sc and Zr solutes and their effect on the mechanical properties of the Al Σ5 (210) [110] grain boundary was carried out by first principles calculations. It is concluded that Sc and Zr both have a powerful driving force to segregate to the grain boundary, and based on the most negative segregation energy, first one Sc atom segregates to the grain boundary, then one Zr atom and finally another Sc atom. The grain boundary energy, strengthening/embrittling energy, fracture energy and theoretical tensile peak stress all demonstrate that Sc and Zr solutes have a strengthening effect on the grain boundary, which can be attributed to the combination of the "chemical effect" (charge density variation) and the "structural effect" (atomic arrangement change), especially the migration of the first Sc-segregated site toward the grain boundary, which not only brings stronger Sc/Zr-Al bonds instead of weak Al-Al bonds but also leads to the shrinkage and charge accumulation of the great vacuum area between two grains. This work sheds light on the underlying mechanism of the better mechanical performance due to the co-addition of Sc and Zr solutes at the atomic and electronic levels.