Nanostructure carbide-free bainite medium-carbon alloy steel comprises the following chemical components by weight: 0.48-0.52 parts of C, 1.2-1.3 parts of Si, 1.7-1.9 parts of Mn, 1.0-1.4 parts of Al, 1.6-1.8 parts of Cr, 0.5-0.6 parts of Ni, 1.9-2.3 parts of W, less than 0.02 parts of P, less than 0.02 parts of S, and the balance of Fe; the preparation method comprises the following steps: melting the medium-carbon alloy steel with the above components by a vacuum induction furnace, casting to form a steel ingot, slowly cooling to room temperature; heating, performing heat preservation, forging after the steel ingot is taken out of the furnace, slowly cooling to room temperature; heating and austenitizing the product, rapidly cooling to a supercooled austenite temperature range, performing compression deformation with a strain rate of 0.01 s-1 and a press quantity of 50%, rapidly cooling to a temperature slightly higher than a martensite transformation starting temperature, performing isothermal transformation, and cooling to room temperature. The carbon content of the alloy steel of the invention is greatly reduced when compared with that of high-carbon steel, which not only facilitates the improvement of solderability and impact toughness, but also shortens the treatment period, and reduces material cost.