In order to solve the problems of elemental segregation, limited properties and high cost in the preparation of high difference of multi-alloys melting point, a new process of accumulative roll bonding (ARB)-diffusion alloying (DA) has been developed. The properties of the ARB-DA Cu-21Ni-5Sn alloy were characterized by SEM, EDS, TEM, XRD and the universal testing machine. The effects and mechanisms of the ARB and step diffusion heat treatment processes on the compositional homogeneity of the Cu-21Ni-5Sn alloy were investigated, and the effects and mechanisms of the subsequent aging regime on the properties of the Cu-21Ni-5Sn alloy were revealed. The results show that Cu-21Ni-5Sn alloy with less than 5% elemental error and homogeneous composition was prepared by ARB 7 passes + 650 ℃/5 h + 1000 ℃/8 h step vacuum diffusion heat treatment process. The Cu-21Ni-5Sn alloy was aged at 470 ℃ for 60 min under 40% pre-cooling deformation for 60 min to fully spinodal decomposition. Dense DO22 and L12 ordered solid solution with matrix with α-Cu matrix was(-1-1-1 )Cu//(-2-20)DO22, (-200)Cu//(-310)L12. The alloy reached tensile strength of 925 MPa, a modulus of elasticity of 135.4 GPa, and an alloy conductivity of 6.23% IACS.