The invention provides a tooth preparation undercut detection device. The tooth preparation undercut detection device comprises a horizontal guide rod, a left arm, a right arm, a slider, a positioningblock, a measuring pin and a positioning pin. One end of the left arm is movably connected with the slider through a connector, the other end of the left arm is connected with an end of the right arm, the other end of the right arm is connected with the positioning block, the positioning block is fixed to one end of the horizontal guide rod, the other end of the horizontal guide rod is movably passed through the slider, the lateral side of the slider is provided with a limiting block, the limiting block is provided with a through hole, the measuring pin is passed through the through hole, anda top plate of the slider is provided with an elastic limiting rod and a rigid limiting rod. The tooth preparation undercut detection device has the advantages that taking a mold to duplicate a modelis not required, time is saved, whether a preparation has an undercut can be quickly detected, and the device is suitable for any tooth site and can be extended to any position with operational spacelimited, such as a posterior tooth; ink marking on the measuring pin enables the position of the undercut in a tooth to be detected and marked, and indicating can be achieved during continual tooth preparation.