The invention relates to a preheating type 3D printing head changing mechanism. The preheating type 3D printing head changing mechanism comprises a printing head, a replaceable printing mechanism anda preheating mechanism; the printing mechanism comprises a printing shell, a discharging nozzle is arranged at one axial end of the printing shell, and an annular concave discharging groove is arranged at a position, corresponding to the discharging nozzle, of the inner wall of the printing shell; the preheating mechanism comprises a preheating shell, a deceleration nozzle is arranged at one axialend of the preheating shell, a preheating head is fixedly arranged at the end part of the deceleration nozzle, and the preheating head comprises a preheating tube made from a heat conduction material; the inner wall, corresponding to the deceleration nozzle, of the preheating shell is provided with an annular preheating sleeve; and the printing head comprises a discharging tube, a feed inlet is arranged at one axial end of the discharging tube, a discharging opening is arranged at the other axial end of the discharging tube, the internal of the discharging tube is connected with a heating ring, a heat conduction rod is fixedly arranged at the middle part of the heating ring through a hollow-out supporting ring, the other axial end of the heat conduction rod is connected with a plurality of scraper plates made from an elastic material through a connecting column, and the peripheries of the scraper plates bend toward the discharging opening of the discharging tube.