A backstop socket structure to prevent a lamp string from turning-off upon a bulb dropped unintentionally, which comprises a fuse-type bulb mounted in a socket of a lamp string and a plurality backstop sockets; each backstop socket is furnished with two contact copper plates, which are furnished with two spring reeds in close contact state normally; the bulb base has a rod stub under the center of the bulb base; after the bulb base is plugged in place, the rod stub would push the two spring reeds separated from each other; in case of a bulb base being dropped or missing, the two spring reeds will be in close contact state as a result of the resilient force thereof so as to keep the lamp string in lighting up state.