The Ar+CO2 gas shielded welding method was used in the condition of non-preheating,and the low-alloy basic slag flux-cored wire was selected to weld Q620 steel under three heat inputs. The weldability and engineering usability of high strength and low alloy Q620 steel were studied by welding joint quality analysis,weld metal chemical composition analysis,microstructure observation and mechanical property test. The results show that Q620 steel has good weldability at room temperature,and the alloying elements in the weld zone are evenly distributed. When the welding heat input changes from low to high,the microstructure of the weld zone changes from ferrite +pearlite to bainite +martensite,and the strength and hardness of the weld zone decrease. With the increase of heat input,the impact fracture morphology of weld zone changes from ductile fracture to hybrid fracture mode of dimple and quasi-cleavage.