As a low carbon martensitic precipitation hardening stainless steel, G520 steel has been widely used in heavy load and corrosion-resistant components such as compressor impeller due to its high strength with reasonable toughness, ductility and corrosion resistance. Although heat treatment usually presents a tendency to promote a improvement of mechanical properties, it may cause unpredictable changes in the microstructure and properties of high strength steel weldment, which is extremely complicated and normally very sensitive to heat. Based on this scenario, the influence of quenching and tempering on the mechanical and microstructural properties of G520 steel weld metals obtained by shielded metal arc welding (SMAW) was studied in this work. Tensile test, impact test and metallographic examination by OM, XRD, SEM and EBSD were performed for mechanical and microstructural characterization. The results indicate that, the welded joints after quenching (at 850 ℃, oil cooling) and tempering (at 520 ℃, air cooling) have better strength and toughness than the pre-weld quenching and tempering. Moreover, the quenching and tempering treatment of the weld metal, breaks down the columnar microstructure into smaller martensite sub-blocks. Meanwhile, it form a certain amount of inversion austenite at the prior austenite grain boundary and the boundary of the lath martensite. As above, the proportion of the large angle grain boundary is increased, which effectively improves the toughness of the weld metal.