Abstract: The effect of NaF content on the stability of CO2-gas shielding arc was studied by using high-speed photograph. The results show that the welding process can be improved by adding NaF into the flux-cored wire. While NaF content is added to 1.5%, the welding process is the best stable, which has the least spatter based on the small particles. The quality of deposited metal is high and welding defects is free on the top surface. Besides, the microstructures and mechanical properties were studied in this paper. The results show that the deposited metal has high hardness because of the high content of martensite and great stability at high temperature. There are few differences of yield strength and tensile strength while the experiments are carried on at the room temperature and 600 ℃. Compared to the as-forged 5CrNiMo steel, the deposited metal has better wear resistance. The experiments show that the wear rate of deposited metal is 18.75%, which is less than that of the 5CrNiMo forged at 600 ℃ and can be used to repair the die.