The invention provides a protective device of tunnel waterproof boards used in the welding of two lining steel bars of a tunneland an operation method thereof. The protective deviceincludes an umbrella supporting guiding device, an umbrella cover and an umbrella cover supporting device which are arranged on one end of the umbrella supporting guiding device, an umbrella supporting protective mat arranged on the outer side surface of the umbrella cover, anda locking device and an adsorption device which are arranged in the middle part of the umbrella supporting guiding device, wherein, the umbrella cover is made of fireproof flexible materials, the adsorption device is fixed on the outer wall at the middle of a slide guiding tube and capable ofsliding back and forth. A magnet is arranged inthe shell of the adsorption device, and the side, in contact with the two lining steel bars, of the shell is shaped as a curved groove. During the welding of the two lining steel bars, the protectivedeviceprotects heat-sensitive materials such as the waterproof boards from high temperature and electric sparks, greatly reduces the damage of the waterproof boards and improves the reliability of thetunnel waterproof system.