The invention discloses a construction welding method for a major side pipe in a pile leg, comprising a plurality of connecting sections composed of a plurality of semicircle plates and rack plates. The welding procedure is described as follows: a. the two side semicircle plates are welded with the rack plate into the form of a unilateral U-shaped groove joint, with the semicircle plate positioned in the middle of the rack plate and shorter than the rack plate; b. the terminals of the two sections of rack plate are welded into the form of an unsymmetrical X groove joint; c. a filling semicircle plate is filled in between two connecting sections of semicircle plates in the form of a V-shaped groove joint through welding. The construction welding method for a major side pipe in a pile leg has the advantages that the accessibility of the welding construction is enhanced while the welding joint is reduced; workload is decreased while work efficiency is enhanced and mechanical properties of the welding joint are improved. After welding procedure evaluating test, every index of the mechanical properties of the welding joint meets the requirements of a classification society.