Without the assisstance of sufactant, CuCl₂ was reduced by NaBH₄ to form Cu nanoparticles, which acted as second dispersant during the preparation of PtCu alloy. The obtained PtCu of 2.1nm was supported on Vulcan XC72. The PtCu/C showed lattice compression, Pt surface segregation, high ratio of non-oxidized Pt with high electronic binding energy. All those structure feature contributed to weak adsorption strength of OHads on Pt, which had benefit on ORR. According to half cell test, the area specific activity and mass activity of PtCu/C reached 6.6 and 3.8 times that of Pt/C at 0.9V vs. RHE. What's more, after degradation test of 10000 cycles, the superiority of PtCu/C towards Pt/C remained. At 100 mA/cm² in single cell test, ultra-small PtCu/C displayed superior performance and stability. Besides, this method in this article can also be used to synthesize supported ultra-small alloy, such as PtCo and PtNi.