The performance of metals in service is significantly influenced by grain boundaries(GBs). This study utilizes density functional theory and employs first-principles calculations to systematically look into the effects of 27 transition metal elements on copper's Σ5 [001](210) grain boundaries. Additionally, first-principles tensile methods are employed to quantitatively assess how alloying elements influence grain boundary strength. The fundamental reason behind the alloy atoms' influence on grain boundary(GB) strength is elucidated from the perspective of charge density variations. This research contributes to a deeper understanding of the behavior of transition metal elements at the Σ5[001](210) grain boundary on an electronic scale, consequently providing a theoretical framework for enhancing the strength of copper.