The invention provides a welding method of an ocean platform module wall board gas shielded flux-cored down weld, comprising the following steps of: preparing a welding joint groove and cleaning; assembling interfaces at the joint of two wall boards; preheating a welded position before welding; and carrying out preparing working procedures such as setting a welding method and welding parameters, setting welding materials and setting shielding gas and parameters, welding working procedure and treatment working procedure after welding, wherein the welding materials in the preparing working procedure before welding are set to be flux-cored welding wire, and the shielding gas is set to be CO2. The welding method has the improvement in that the welding joint groove in the preparing working procedure before welding is I-shaped, and the welding way is set to be vertical down weld. The invention can improve the welding speed, achieves attractive welded joint formation, greatly lowers the welding distortion, reduces the workload for wall welding deformation correction after welding and welded joint polishing, and efficiently improves the working efficiency.