Abstract: The rail fastenings fractured when they were used in a humid tunnel about two years. The fracture reasons were analysed with means of macroscopical observation，scanning electronic microscope and X ray energy spectrometer，metallography microscope and so on. The result shows that environment is stronger acid and the fastenings are seriously rusty，a small quantity of Cl and S element is detected in the corrosion. The fracture origin lies in the symmetric inner arc where the tensile stress is ultimate and the narrow gap happened because of serious corrosion. The shell pattern is apparent ，so the fastenings failure belongs to corrosion fatigue cracking. It can be prevented by using better corrosion resistance steel，increasing tempering temperature properly，hot galvanizing and reducing the fastenings serviceable life.

Key words: corrosion fatigue cracking, fracture origin, corrosion, tempered martensite