The invention discloses a welding device of a main side pipe and a welding method thereof. The device and the method are used for main side pipe welding of half chord pipes in left-right asymmetry of a large-sized self-elevating platform pile leg. The welding device of the main side pipe comprises a welding pedestal, a clamping and fixing device and a horizontal adjustment device, wherein the welding pedestal is used for supporting the to-be-welded main side pipe; the length of the welding pedestal is greater than or equal to the length of the to-be-welded main side pipe; the clamping and fixing device is arranged on the welding pedestal for clamping and fixing the to-be-welded main side pipe; the horizontal adjustment device is used for horizontally adjusting the to-be-welded main side pipe. The invention also discloses a method for welding the main side pipe with the half chord pipes in left-right asymmetry of the large-sized self-elevating platform pile leg by using the welding device of the main side pipe. According to the method and the device, the welding quality, the dimensional accuracy, the operation efficiency and the economic benefit of the half chord pipes in left-right asymmetry are improved, and the machining requirements on straightness, camber and twist in the welding process of the main side pipe in left-right asymmetry are met.