Abstract: Q345B low carbon steel and 20Mn23Al non-magnetic steel were welded by gas metal arc welding with E316LT0-1 stainless steel welding wire. The microstructure and mechanical properties of the welded joint were studied. The results show that the microstructure of the filling weld layer consisted of skeletal δ ferrite and austenite, and that of the backing weld layer was composed of dendritic ferrite and austenite. The microstructure of the overheated coarse-grained zone on the Q345B steel side consisted of bainite and Widmanstaten structure, and that of the phase transformation recrystallization zone consisted of pearlite and ferrite. The heat-affected zone on the 20Mn23Al steel side had a microstrucrture of twin austenite and a small amount of ferrite. The interdiffusion of alloying elements in the base metals and weld occured, and the diffusion layer thicknesses on the Q345B steel side and 20Mn23Al steel side were about 15, 25 μm, respectively. The tensile property of the joint was better than that of the Q345B steel while worse than that of the 20Mn23Al steel. The weld of the joint had the largest hardness, followed by the heat-affected zone.