The invention discloses copper hardened steel added with nanoparticles and a preparation method of the steel. The copper hardened steel comprises the following components: nano aluminum oxide, copper, silicon, carbon, manganese, aluminum, nickel, phosphorus, oxygen, nitrogen, sulfur and the balance of iron. The method comprises the following steps: refining a mixture of oxygen-free copper and nano aluminum oxide particles in a vacuum melting furnace, and melting to form an intermediate alloy under the conditions that the pressure is 5.0*10<-3>Pa and the electromagnetic stirring action is applied; preparing alloy according to a ratio of the components in the claim 1, adding the intermediate alloy into the prepared alloy, and refining in the vacuum melting furnace to obtain a casting blank; deforming and tempering to obtain hardened steel with uniformly distributed hardening phases-copper. The copper hardened steel is excellent in strength, flexibility, thermal forming property and mechanical property.