Abstract: Welding efficiency is the key to ocean platform and hull construction.Welding efficiency can be increased and the cost of construction can be reduced by no preheating welding.Two kinds of deposited metal of 690 MPa high strength low alloy steel with different Mo content were prepared by shielded metal arc welding (SMAW)without preheating.The relationship between microstructure and mechanical properties was analyzed.The results showed that Mo content had obvious effect on microstructure,phase transformation,plasticity and strength-toughness properties.With the chemical composition system of 0.02%～0.03%C,4.4%～4.5%(Mn+Cr+Ni),0.23%～0.60%Mo,refined microstructure with good plasticity and high strength-toughness was obtained by controlling Mo content.Fine acicular ferrite with good plasticity could restrain the initiation and extension of crack,which could obviously increase the impact toughness.However,the mechanism of improvement on the toughness by microstructure refinement was obviously different from that of increased plasticity.The density of grain boundary could be increased by the refined microstructure,which could hinder the extension of crack.Microstructure with good plasticity can restrain the initiation of crack and hinder its extension.The deposited metal with tensile strength of 742 MPa,-50 ℃ impact absorbed energy of 82～114 J (average value 103 J)was obtained with the chemical component of 0.03%C,4.4%(Mn+Cr+Ni),0.23%Mo without preheating.Highlights:(1)Effect of Mo element on microstructure,strength and toughness of the deposited metal in the composition system of 0.02%～0.03%C,4.4%～4.5% (Mn+Cr+Ni)and 0.23%～0.60%Mo and mechanism were studied without preheating.(2)Fine deposited metal with good plasticity and high strength toughness was obtained by controlling Mo element,influence mechanism of microstructure refinement,plasticity on toughness of deposited metal was illustrated by fracture observation.