Abstract: In order to improve the welding efficiency of structure members, it is of great significance to adopt large penetration welding. The welding test of Q355B low-alloy high-strength steel for tunnel structure with a thickness of 30 mm was carried out by using the method of deep penetration welding, and the microstructure and mechanical properties of the welded joint were tested. The results show that the microstructure of deep penetration weld is mainly composed of a large number of ferrite and a small number of granular bainite; the penetration depth of large penetration welded joints is significantly increased, which greatly improves the welding efficiency; the microhardness of the weld with large penetration is close to that of the base metal, and the whole joint is an equal strength matching; with the increase of welding current, the microhardness and tensile strength of the weld have little change, and the impact toughness decreases; the best current range for deep penetration welding is 320 A-360 A.