Abstract: The leakage reasons of hot dip galvanized steel pipes used in central air-conditioning water circulatory system were analyzed by means of chemical compositions analysis, macroscopic examination, metallographic examination and energy spectrum analysis. The results show that the leakage of the pipes was caused by pitting corrosion which formed in the welded area on inner wall of the steel pipes. The main reason for the corrosion was the existing of corrosive medium such as dissolved oxygen and sulfide ion, and the direct cause for the corrosion was due to deletion of necessary second anti-corrosion treatment for the pipes.