A protective device includes a substrate, two first electrodes, a low-melting point metal layer and an assisting layer. The first electrodes are respectively arranged at two opposite sides of the substrate. The low melting point metal layer is arranged over the two first electrodes. The assisting layer is formed on the low melting point metal layer. The liquidus temperature of the assisting layer is below the liquidus temperature of the low melting point metal layer, and the liquidus temperature of the assisting layer is not below a predetermined temperature which is below the maximum working temperature of reflow soldering process by 25 degrees.