The invention discloses a thin-wall stainless steel welding protectant and a preparation method, which are used for solving the problem of thin-wall stainless steel welding. The technical scheme of the invention is that: the welding protectant is prepared from the following materials by weight: 500g of water glass, 100 to 117g of titanium dioxide, 60 to 72g of fluorite powder, and 50 to 70g of feldspar powder. The welding protectant in the invention is coated on the back side of a thin-wall stainless steel welding line before welding, the welding is performed on the front side, therefore the welding line is flat and is formed completely on the two sides of the welding line after the welding, and welding defects, such as welding beading, welding slag and the like, are not produced. By using the welding protectant in the invention, a complicated procedure of using an argon chamber on the back side of a welded piece can be abandoned, so that labor resources, financial resources and material resources are saved, welding quality and working efficiency are greatly improved, and production cost is reduced.