Abstract: The corrosion behavior of 110SS carbon steel tubing and 140ksi carbon steel casing in polysulfonate drilling fluid and the galvanic corrosion behavior of simulated downhole tubing and casing contacting with each other were studied by weight-loss corrosion test. The results show that in the polysulfonate drilling fluid at 120 ℃ and a density of 2.35 g/cm~3, the corrosion rate of the single 110SS carbon steel tubing and 140ksi carbon steel casing is extremely slight, which is 0.0078mm/a and 0.0064 mm/a, respectively, and the maximum pitting depth is 11.9 μm and 4.0 μm, respectively. After 10 days and30 days of coupling corrosion, the corrosion rates are 0.0054 mm/a-0.0049 mm/a and 0.0033 mm/a-0.0032 mm/a, respectively,which belongs to slight corrosion and the corrosion rates decrease with the extension of test time, secondly, the difference of the corrosion rates between the two decreases gradually. The pitting rate is 0.1095 mm/a-0.2482 mm/a and 0.3906 mm/a-0.0462 mm/a, respectively. The pitting corrosion rate of 110SS carbon steel increases and 140ksi carbon steel decreases. The corrosion products mainly contain Fe_x-1S_x and SiO₂ attachments, which shows that the galvanic corrosion effect between 110SS carbon steel tubing and 140ksi carbon steel casing is very small, which is not reflected in the form of uniform corrosion but pitting corrosion.