The invention discloses an internal thread detection device and method and discloses a method for performing measurement and reconstruction on the upper surface and the lower surface of a thread respectively, and then thread parameter measurement can be achieved. Due to the fact that only the upper surface or the lower surface of the thread is measured each time, it is not necessary for a camera to capture a complete tooth form image at a time, the camera and a laser projector can be arranged outside a detection hole, and the limitation on the size of equipment is avoided. Through the method, complete shape information of the internal thread is obtained finally, and foreign substance examination and measurement of the depth, the pitch and the tooth form parameter of the thread can be carried out simultaneously. Because the majority of threads are made of metal, projection light performs specular reflection on a processing surface of the thread, the camera and the laser projector are arranged on the same side of the detection face, and therefore the camera can obtain clear projection light images beneficially; in addition, an automated surface splicing technology based on the optimization theory is put forward, the splicing accuracy of the upper surface and the lower surface of the thread is improved, a program can be implemented easily, automation of subsequent data processing is improved, and measurement operations are simplified.