The invention discloses a mixing device for preparing a consumable of a 3D (Three-Dimensional) printer. The mixing device for preparing the consumable of the 3D printer comprises a bottom plate; the upper surface of the bottom is fixedly connected with the lower surface of a first box body through two fixed rods; and a stirring device is arranged in the first box body. According to the mixing device for preparing the consumable of the 3D printer, through the cooperation among a second box body, a material outlet, a baffle plate, a sliding block, a sliding groove, a threaded cap, a threaded rod, a second rotating shaft, a driven gear, a driving gear and a second motor, an output shaft of the second motor rotates forwards to drive the baffle plate to move away from the position below the material outlet, and the output shaft of the second motor rotates reversely to drive the baffle plate to return to cover the material outlet, so that the amount of the consumable which is poured into thefirst box body can be regulated automatically; a material barrel is not required to be held by people all the time when the consumable is poured into the first box body; the labour intensity of the people is lightened; the time and physical power of the people are reduced; and therefore, convenience is brought to the people when the consumable of the 3D printer is mixed.