A Cu−1.9Ni−1.9Co−0.9Si (mass fraction, %) alloy with high strength and electrical conductivity was designed by cluster formula approach. The microstructure evolution of the alloy during thermomechanical treatment was systematically investigated. The strengthening mechanism and electrical conductivity of the alloy were discussed in detail. The optimal thermomechanical treatment process was as follows: solid solution → 80% cold rolling → (450 °C, 4 h) aging → 50% cold rolling → (400 °C, 4 h) aging. The designed alloy achieved excellent comprehensive properties with a microhardness of HV 260, a yield strength of 843 MPa, a tensile strength of 884 MPa, and an electrical conductivity of 42.6%(IACS). Compared to direct aging treatment, the designed alloy subjected to multi-stage thermomechanical treatment had refined grains, high density of dislocations, and accelerated of precipitation of (Ni,Co)₂Si precipitates. High strength was mainly attributed to the combined effect of dislocation strengthening, work hardening and sub-grain strengthening, while good electrical conductivity was maintained through the precipitation of the large number of nanoparticles.