Failure analysis of a 330 MW subcritical unit high temperature reheat steam pipe inwall cracks was studied by means of macroscopic observation, metallographic examination, composition analysis, hardness test and structure analysis. The results showed that the cracks in the inner wall are thermal fatigue cracks, which was caused by the repeatedly change of ambient temperature around the drain pipe hole. When the expansion and contraction of pipe wall caused by temperature variation is restrained, cyclic thermal stress was produced. In this way, the material is subjected to fatigue damage, and the transverse crack defects perpendicular to the expansion and contraction direction of the pipe are formed.