The as-cast structure, solid solution and aging characteristics and structure changes of Cu-15 Ni-8 Sn alloy with Mg content of 0.05%～0.30% were investigated. The experimental results indicate that Mg is segregated in the Sn-rich phase at as-cast state, and significantly inhibits the discontinuous precipitation during the alloy aging process, thus improving the mechanical properties. When the ageing time is 6 h, with the increase of Mg content, the alloy hardness is increased and the electrical conductivity is decreased. The hardness and conductivity of the alloy with 0.3% Mg are 369 HV and 4.85 MS/m, respectively, which are increased by 11 HV and decreased by 0.26 MS/m compared with those of one without Mg.