Abstract: The weldability of D6AC ultra-high strength steel is analyzed, and the fundamental of tandem sequence submerged arc welding with thin-wire is introduced. Welding tests are carried out to study the procedure of D6AC ultra-high strength steel welding with the tandem sequence submerged arc welding technology, and the character of welding ultra-high strength steels by use of this method is analyzed. With the thin-wire tandem sequence submerged arc welding technology, the cracking problem existing in the welding of D6AC ultra-high strength steel can be solved, the welding automation can be realized at low cost, and the low efficiency of TIG pulse welding and the high cost of high-energy density welding are avoided.