NM450 wear-resistant steel plates were welded with ER50-6 welding wire by CO2 gas-shield welding. The microstructure, mechanical properties and welding cold crack sensitivity of the joint were studied. The results show that the microstructure of the weld was composed of massive ferrite and needle-shaped ferrite, and that of the coarse-grained heat-affected zone and of the normalized zone was composed of lath martensite and ferrite+cementite, respectively. The hardness of weld metal, coarse-grained zone, normalized zone and incomplete recrystallization zone was 220, 412, 234, 386 HV, respectively. The decomposition of martensite led to the decrease of hardness in the normalized zone and incomplete recrystallization zone. The tensile strength of welded joint was 768 MPa. The impact energy of weld center, heat-affected zone and base metal at -20℃ was 110, 140, 88 J, respectively. The steel had a certain cold crack sensitivity. No cracks were produced for the welding at 32.6℃ without preheating. Both the root crack percentage and cross-sectional crack percentage were 100% for the welding at -1.4℃ without preheating, while no welding cracks were produced after preheating at 80℃.