Abstract: The experiments of special flux assisted TIG welding-brazing for aluminum and steel dissimilar alloys was carried out by using Al-Si eutectic filler metal. The special flux applied to TIG heating was developed, the wetting and spreading behavior of liquid filler metal on the steel surface was observed by high-speed camera, and the typical TIG welding-brazing butt joint was analyzed. The results show the special flux can greatly promote the wetting and spreading of the filler metal on the steel surface, which the composition (mass fraction,%) is the modified Noclock flux (KAlF₄:K₃AlF₆, 65:35) 55-78,Zn 10-20,Sn 10-20 and K₂SiF₆ 2-5. With the action of special flux, the spreading behavior of liquid filler metal consists of two parts:one is to spread on the back face to pack the steel; the other is to uphill spread on the front face of the groove to form a sound brazed joint. A sound welding-brazing butt joint is obtained, which has a dual characteristic:in aluminum side it is a welding joint, and in steel side it is a brazing joint. The tensile strength of the butt joint reaches 115.5 MPa and the fracture occurs the interface of welded seam and the steel.