According to the first principles methods, we theoretically study the mechanical properties of He doped AlΣ3 grain boundary (GB). Our results show that He has the lowest formation energy 2.942 eV in GB and low segregation energy 0.085 eV from bulk to GB. Under the extending tensile, the clean GB has a theoretical strength of 8.95 GPa, and the crack appears first from the GB. While, after the He doping, the tensile strength reduces to 7.14 GPa, and one flat effect is present in the curve of strain-stress. By analyzing the variation of the bond-length and the charge distribution, we believe that He-induced reduction of tensile strength is probably ascribed to the weaker interaction between He and Al atoms as well as the weakening Al-Al bonds, owing to the charge screening by full-shell electron structure of He.