Bimetallic alloy nanocrystals (NCs) have received great attention because their physical and chemical properties greatly depend on their composition and structure. However, simultaneous control of the composition and surface structure (or morphology) of metal alloy NCs is challenging due to differences in standard redox potential (SRP). According to our previous research, well-shaped and composition-tunable Pt–Cu alloy NCs can be controlled in mild reduction conditions. However, the content of Cu in the as-prepared Pt–Cu alloy NCs cannot exceed 50% using N,N-dimethylformamide (DMF) as the weak reductant. In this paper, we reported a successful synthesis of the octahedral Pt–Cu bimetallic alloy NCs with a molar ratio of Cu ranging from 50% to 75% via introducing a slightly stronger reductant (n-butylalcohol) to improve the reducing ability and tune the sequential reaction kinetics. It is found that n-butylalcohol can solely reduce the Cu precursor to metallic copper. The UPD-like process ensures the formation of the Pt–Cu alloy due to the strong binding energy between Cu atoms and the Pt crystal surface. Owing to the successful control of Cu, the content in Pt–Cu alloy increased from 50% to 75%, and the relationship between the composition and the properties of the electro-catalytic oxidation of formic acid were further investigated. The results reveal that the electro-catalytic performance of the Pt–Cu alloy is enhanced along with Cu content, and that PtCu₃ exhibits excellent catalytic activity and anti-poisoning ability.