Abstract: This paper holds that the key to shielding during welding of titanium alloy sheets lies in the size of heating zone above 700℃ and the durarion of cooling to 400℃. The authors proved that it is rational to regard the oxide film as the mark of contamination of titanium ]alloy sheets and the the thickness of the oxide film as the criterion of the degree of contamination. Electron microprobe analyser was used to measure the thickness of the oxide film, and the equation relates the thickness of oxide film with heating temperature and holding time was obtained through data processing. Therefore, the degree of contamination during welding o’f titanium alloy sheets can be predicted and controlled through selecting welding methods and setting welding parameters.