980MPa grade high strength low alloy steel was welded by ultra-narrow gap welding. Observed by optical metalloscope, there were large majority of equiaxial crystals in weld centers, which was the weld cross-section of 18.3%. Each weld could be compartmentalized into original microstructure zone, over heated zone and normalizing zone under next weld thermal cycle, in which the microstructures were almost the same. Through quantificational metallographic analysis, it was found there was more than 98.9% of acicular ferrite in weld metal, and other microstructures were fine. Mechanical test indicated that the strength of weld metal was higher than body material, and had outstanding plasticity. What is more, the toughness of weld metal was better than solder wire.