Abstract: The strengthening and toughening of weld metal is a technical difficulty of ultrafine grained steel welding, and it needs to develop a kind of welding material matching the base metal in order to realize the strengthening and toughening of welded joint and avoid the cold crack.The ideal strength and toughness can be obtained by alloying mathod and controling weld structure to obtain the acicular ferrites.Through experiments, and unifying the microstructure and mechanical properties characteristics of 400 MPa grade ultrafine grained steel, a special-purpose electrode of 400 MPa grade ultrafine grained steel was develeped.The examination results show that, the microstructures of weld metal by this electrode are tiny acicular ferrites, and the yield strength and teusile strength of the welded joint are 435 MPa and 612 MPa, respectively.The impact absorbing energy is 148 J.Its microstructures and mechanical properties match the 400 MPa grade ultrafine grained steel well, and achieve the anticipated goal.