Abstract: An unloading device and an unloading method for a gantry-type machining center beam guide rail is disclosed, which is used to be installed on a sliding plate assembly, comprising a first booster mechanism and a second booster mechanism. When a worm rotates and drives a worm gear to rotate, an axial displacement can be generated by the worm gear along a worm gear shaft to push the retaining sleeve assembly, and an unloading force generated acts on a first mounting bracket. An unloading bolt of the second booster mechanism is connected with a press plate through threads, the unloading bolt is rotated to generate an axial displacement, and a second sliding block is pressed tightly against an unloading guide rail vertical surface by a second mounting bracket to generate an unloading force.