A detecting mechanism of a disc pick-and-place device comprises a first frame and a pick-and-place head, a detecting portion and a sensor respectively coupled thereto. When the pick-and-place picks up at least two stacked discs, the detecting portion then shows a state different from when the pick-and-place head picks up one disc, and when the sensor senses that the detecting portion shows the different state, it may then output a corresponding signal thereby allowing the disc pick-and-place device to detect whether at least two discs are picked up and be prevented from placing at least two discs into a disc copier to avoid the incapability of copying and the speed decreasing of copying and even the damage of the copier.