The allocation of laser power was a complicated process for narrow gap laser welding with filler wire. The narrow gap laser welding of a 16 mm thick 9%Ni steel plate was performed with nickel based alloy as filler wire. The effects of laser power, defocusing amount, and the distance between laser and filler wire were investigated through orthogonal experiments. The allocation paths of laser energy on molten pool, keyhole, metallic vapor, and filler wire were presented. The influencing mechanism and optimization strategies of sidewall fusion ratio, coarse grain region width, and weld bead surface evenness were investigated.