A position detecting device of an optical drive utilizes a simple wire to connect a CPU of the optical drive to a latch part and a fixing part, such that the latch part, the CPU and the fixing part, which are original parts of the optical drive, form a detecting circuit. When the latch part clasps or releases a tray, the latch part latches or releases the fixing part to close or open the detecting circuit so that the CPU can correctly judge whether the tray enters for positioning, and thus start or shut down the optical drive. In addition, the members such as a flexible cable, a casing and a mainboard may replace parts of the wire to simplify the detecting circuit.