Abstract: The shielding gas plays an important role in welding, and its composition has a great influence on droplet transfer and metal flow in molten pool. A 9Cr-1Mo coating was prepared on the surface of Q235A steel by cold metal transition (CMT) technology. The effects of pure Ar, 95%Ar+5%O₂ and 80%Ar+20%CO₂ on the formation, dilution rate, microstructure and properties of the coating were studied.The results show that the formability of the coating is the best under the condition of 95%Ar+5%O₂, and there is an obvious undercut at the weld toe under pure Ar condition, while there is obvious spatter phenoumenon in the protective atmosphere of 80%Ar+20%CO₂. Under the condition of 95%Ar+5%O₂ protection, the average dilution rate of the coating is 25.08%, and the metallurgical bonding between the coating and the substrate is good, and the performance degradation caused by excessive dilution of the coating metal is avoided. The microstructure of the coatings under the three protective gases is lath martensite, and the microstructure of heat affected zone is composed of quenched martensite, pearlite and ferrite, but the martensite content under the 95%Ar+5%O₂ shielding gas is obviously lower than that under the other two protective atmospheres, and the grains are finer.