The invention relates to a steel-making material on-line mixing preheating device and method. A raw material bin bottom discharging opening of the device communicates with a mixing preheating chambertop feeding opening through a weighing mechanism, an unloading valve and a conveying pipe in sequence, and a vibrating screen is arranged in a raw material bin; a plurality of guide inclined plates are arranged inside a mixing preheating chamber in the height direction, and the adjacent guide inclined plates are arranged in stagger; the exterior of the mixing preheating chamber is wound with a heating coil; an oppositely-opening type material door is installed at a mixing preheating chamber bottom discharging opening; and a discharging chute is arranged below the mixing preheating chamber in an inclined manner, and the discharging end of the discharging chute communicates with a steel refining furnace opening. The method comprises the steps that the vibrating screen is started to enable materials to fall to the weighing mechanism, and the vibrating screen is closed after the weight of the materials reaches a set value; an unloading valve is opened to enable the materials to enter the mixing preheating chamber; the heating coil is started to heat the interior of the mixing preheating chamber, mixing and preheating of the materials are completed in the falling process, and the heating coil is stopped after all the materials fall to the material door; and the material door is opened to enable the mixed and preheated materials to be discharged into the steel refining furnace through the discharging chute.