Abstract: An analysis is conducted on the causes for the pitting corrosion of the P110S tubing in the environment of solid-free well killing fluid. The alloy composition and microstructure of the tubing are confirmed normal via relevant testing. Owing to analyzing the chemical properties and the corrosion products of the tubing as identified before and after the injection of the well-killing fluid，it is determined that some acidic substances are mixed with the well killing-fluid as being injected into the well，and there exists in the corrosion products FeCO₃，a product with CO₂ corrosion characteristics. Finally thanks to the hi-temperature，hi-pressure corrosion simulation test to reproduce the tubing pitting corrosion phenomenon，it is revealed that the above mentioned pitting corrosion of the P110S tubing is caused by the CO₂ as dissolved in the well-killing liquid. Accordingly it is suggested that when selecting proper tubing as working string for a well which operation conditions are not so clear，tubular goods with higher corrosion resistance according to the in-well working conditions as identified before should be used so as to increase the corrosion redundancy of the string for avoiding any potential safety hazard and economic loss as caused by the failure of corroded string.