Narrow-gap submerged arc flux used for nuclear power SA508 Gr.3 Cl.1 steel welding was introduced on slag system selection,formula designation,and product performance. It was welded with the special alloy wire in wide welding parameters,their welding process performance adaptability and stability of mechanical properties in PWHT were researched. The results showed that,flux CHF112 HRF had excellent welding process performance adaptabilities. When welded variably in wide welding currents,welding speed,interpass temperatures,mechanical properties of deposited metal in PWHT met the technical requirements of the third generation nuclear power completely,and the values had sufficient margins and less fluctuations. The microstructure of deposited metal was granular bainite and ferrite.