A hole expanding device includes an electric heating device, a moving device, an rotating device, and a carrying device. The electric heating device includes a transition portion and a hole expanding rod fixed onto the transition portion. The electric heating device heats the hole expanding rod to a predetermined temperature. The rotating device is fixed onto the moving device. The moving device drives the rotating device to move along a direction parallel to the hole expanding rod. The carrying device is fixed onto the rotating device and includes a receiving groove for receiving and fixing a workpiece. The rotating device rotates the carrying device to make two opposite surfaces of the workpiece face the hole expanding rod.