The microstructure and mechanical properties of P355NL1 steel MAG welded joints before and after stress relief annealing treatment were compared. The results show that a welded joint with good appearance could be obtained by the MAG welding method. The stress relieving annealing treatment has no obvious effect on the microstructure of the weld,heat-affected zone and base metal of the welded joints.The stress relief annealing treatment has little effect on the tensile properties,breaking elongation and bending properties of the welded joints,but the hardness of the joint reduces to a certain extent. The impact absorption energy of the stress relief annealing joint weld has little change,while the impact absorption energies of the base material and heat-affected zone have a certain degree of improvement. On the whole,the stress-relieving annealing treatment has no significant influence on the microstructure and mechanical properties of the P355NL1 steel MAG welded joints,which can ensure the good service performance of welded joints.