The invention discloses a welding technique for a base plate of an LNG low-temperature tank and belongs to the technical field of welding of LNG low-temperature tanks. According to the welding technique, two 9Ni steel plates with the thicknesses ranging from 6 mm to 9 mm are made to be in lap joint in the horizontal direction and overlap to form an overlap joint, the end face is cleaned, a preparation for welding is made, and then submerged-arc welding is conducted by means of alternating current, wherein the arc voltage ranges from 25 V to 29 V, the welding current ranges from 260 A to 290 A, the wire extension length ranges from 25 mm to 35 mm, the welding speed ranges from 22 in / min to 25 in / min, the welding wire angle ranges from 55 degrees to 65 degrees, and a full fillet weld with a welding foot with the length ranging from 6 mm to 9 mm is formed. By the adoption of the welding technique, submerged-arc welding of the base plate of the LNG low-temperature tank can be achieved, the problems that during submerged-arc welding, a lap fillet weld is poorly formed, hot cracks are formed during welding, and burning through occurs easily during welding are solved, the two 9Ni steel plates with the thicknesses ranging 6 mm to 9 mm can be welded in a single-pass mode at a time during actual operation, and welding efficiency is remarkably improved.