The invention discloses a welding method for a natrium medium ultra-low carbon stainless steel tube. The method comprises the following steps that the tube is cleaned and assembled, a backing run is welded, the key point is that after the welding work is finished, the deposited metal at the joint of the backing run is ground but not worn out, and finally the ground place is welded again. The welding method can guarantee the cleanliness of the tube and quality of welding seam, and has the advantages of simple technique, and easy and convenient operation.