The invention discloses a protecting device of a welding seam of a titanium-steel composite plate of a sleeve-type chimney and a use method of the protecting device. After the titanium-steel composite plate of a smoke exhaust inner barrel of the sleeve-type chimney is subjected to edge rolling forming and is assembled, and before the titanium-steel composite plate is butted with a V-shaped welding seam, the protecting device is manufactured on a welding seam titanium plate surface by adopting galvanization C-shaped steel, and when a steel plate sealing welding seam of the titanium-steel composite plate is welded, the C-shaped steel of the protecting device is attracted to the titanium-steel composite plate by using a mightiness magnet on the welding seam titanium plate surface; when a steel plate back surface of the titanium-steel composite plate is subjected to back gouging, a thin steel plate at an opening of the C-shaped steel is removed, and the steel plate sealing welding seam can be subjected to the back gouging by using carbon arc air gouging along a strip-shaped notch. Since space between the C-shaped steel and the welding seam titanium plate surface is filled with a rock wool material, when the steel plate sealing welding seam and the steel plate back surface are subjected to the back gouging and the welding, pollutants like welding slag, splashes and smoke are not in contact with the titanium plate, and the disadvantages that weld cracks are generated and the corrosion resistance is lowered due to the fact that titanium steel interfusion occurs or titanium steel is polluted by iron during the welding can be avoided. The protecting device is simple in structure and easy to use, the construction quality is improved and the construction efficiency is improved.