Abstract: In this paper,400MPa ultrafine grained steel is welded by intelligent spot welder.The joint's microstructure and hardness are studied.The results indicate that the nugget that adopted the spot welding parameters,which are suitable for general low-carbon steel,has shrinkage cavity.The weld defect is conquered by forging force.In the nugget,the microstructure are equiaxed grain and epitaxial grain.In the heat affected zone,the microstructure are overheated structure and secondary solidification structure.The nugget is greatly harder than the base metal,and a little harder than the metal in HAZ.The martensite in the water cooling nugget leads to the former phenomenon,and the latter conclusion is the reciprocal result of welding residual stress and secondary ultrafine grain.