Abstract: TIG back welding in position welding is one of the key manufacturing technology for large rotor in steam turbine. The foreign companies consider it as Know-How and blockade it strictly. In this thesis, carries out welding parameters as arc voltage, welding current,pulse frequency and wire feed speed, and that effects on one-side welding with back free formation of horizontal pulse TIG. The results showed that, the welding current has a vital important influence on penetration resistance of one-side welding with back free formation and followed by arc voltage; pulse frequency for positive impact is relatively large, and can effectively inhibit the tendency to drip; wire feed speed and arc voltage can play an important role in the back weld forming, when the welding parameters is small, the back weld can not be forming.