The invention discloses a high-strength face-centered cubic structure medium-entropy alloy and a preparation method thereof. The CoNiCu medium-entropy alloy comprises Co, Ni and Cu elements; a resultof respective comparison of different medium-entropy alloys in electron concentration and mixing enthalpy according to the Gibbs free energy and the phase formation law of the alloy shows that the CoNiCu medium-entropy alloy has the tendency to form a single-phase FCC structure, a CoNiCu medium-entropy alloy model is constructed based on the tendency, and the alloy is predicted to be a ductile material by a first-principles technique. The preparation method of the alloy comprises the following steps: batching, vacuum melting, suction casting, homogenization annealing and solid solution treatment. Co, Cr, and Cu with a purity of 99% or more are selected and are proportioned according to an equimolar ratio or an approximately equimolar ratio, the proportioned raw materials are placed in a vacuum smelting furnace and are multiply smelted, suction casting molding is carried out after the components are uniform, and the obtained casting undergoes homogenization annealing and solid solutiontreatment to obtain the CoNiCu medium-entropy alloy having a single face-centered cubic structure and having a room temperature compressive strength of above 1600 MPa and a compression ratio of above20%.