Abstract: The corrosion of galvanic couple composed of 17-4 stainless steel and C110 steel was investigated by methods of electrochemical testing, weight loss testing of corrosion, corrosion morphology observasion in simulated casing annulus environment in gas wells where CO2 and H2S dissolved in formation water. The results show that serious galvanic corrosion was found when 17-4 stainless steel coupled with C110 steel. The atomic ratio of Fe to S in galvanic corrosion scale on the surface of C110 steel increased, compared to separate corrosion of C110 steel, leading to the dropping of corrosion resistance of the corrosion scale, and then the C110 steel as anodic material in the couple accelerated the corrosion with the galvanic corrosion factor of 2.51 and the corrosion rate of 0.642 0 mm·a-1. The galvanic current was controlled by the potential difference between 17-4 stainless steel and C110 steel. The potential difference between the two metals in simulated formation water plus annulus protection fluid decreased drastically, and the galvanic corrosion was mitigated markedly. The corrosion rate of C110 steel reduced to 0.042 3 mm·a-1.