Abstract: Macroscopic observation, chemical composition analysis, scanning electron microscopy analysis, microhardness test and finite element simulation were used to study the causes of cracks on the inner wall of the header and drain hole of a super high pressure boiler reheater. The results show that the main reason for the cracks in the header was the hydrophobic reflux of the reheater caused by the unstable operation state of the unit and the unreasonable setting of the drain pipe. Finally, improvement measures were proposed, which had certain guiding significance for ensuring the safe operation of the unit.