The invention discloses a preparation method of medium carbon alloy steel (Q345A-D/Q345X) and relates to the field of metallurgy continuous casting. The preparation method comprises the steps that a secondary cooling water distribution model is optimized, the content of ALx and the content of N are controlled accurately, the content of the ALs is controlled to be no more than 300ppm, the content of the N is controlled below 400ppm, the conical degree of a narrow surface of a crystallizer is adjusted, the conical degree value is adjusted from 1.0% to 1.1%, the accuracy of a device is improved, the device inspection is improved, and the condition of the device is guaranteed to be good. According to the medium carbon alloy steel, transverse corner crack of the medium carbon alloy steel is controlled effectively, the occurrence rate of the transverse corner crack is reduced to 0.92% from the initial 12.5%, the chamfering amount of the casting blank is reduced greatly, and the surface quality of the casting blank is improved at the same time.