Abstract: The weld metal of 690 MPa level high strength low alloy (HSLA) steel was investigated in this paper.The M-A constituents were etched with Lepera reagent,and the microstructure, residual austenite and grain boundary in the welded metal were observed with optical microscope (OM),transmission electron microscope (TEM) and eletron back-scattering diffraction (EBSD),and the impact property was tested.The results reveal that the average Charpy impact values of welded metals made with shielded metal arc welding process and metal inert gas (MIG) welding process were 31 J and 98 J at -50℃ respectively.The volume fractions of M-A constituents in welded metals made with shielded metal arc welding and MIG welding processes were 4.4% and 2.52%,respectively.The increase of volume fraction of M-A constituents resulted in the deterioration of impact property of welded metal with shielded metal arc welding, and the decrease of residual austenite and large grain boundary in the welded metal also led to the degradation of impact property.