The production-well tube string in underground coal gasification (UCG) transports mixed coal gas and suffers corrosion due to high-temperature CO2, H2S and water vapor. Corrosion tests of 9Cr and 13Cr materials in simulated UCG environments between 150 °C and 300 °C were carried out by a high temperature and high pressure autoclave. The effects of gas partial pressure and temperature on the corrosion of the two materials were compared and analyzed. The results show that the most important factor affecting the corrosion of well tubing is temperature, followed by CO2 partial pressure. As the temperature increases from 150 °C to 300 °C, the corrosion rates of both materials first increase and then decrease. When CO2 partial pressure is 3 MPa, corrosion rates of 9Cr and 13Cr materials reach the maximum at 250 °C, which are 1.620 mm/a and 0.728 mm/a respectively.