A Cu-1.3Ni-1.2Co-0.7Si-0.3Cr-0.1Mg alloy with high strength and medium conductivity was designed and prepared. The effects of multistage thermomechanical treatment on the microstructure and properties of the alloy and its strengthening mechanism were studied. The results showed that a large number of nanoscale β-Ni₃Si, δ-(Ni,Co)₂Si and Cr phase particles precipitated in the high-density dislocation region and sub-grain boundaries during aging, and submicron Cr₃Si phase particles formed during solidification of the alloy. The tensile strength, yield strength and electrical conductivity of the alloy were 882 MPa, 857 MPa and 46.7% (IACS), respectively. The main strengthening mechanisms of the alloy were precipitation strengthening, strain strengthening and sub-structure strengthening through the strengthening model calculation and measured strength.