Abstract: In the present work, the cracking of a SA213−T23 steel water-cooling wall pipe in a power plant boiler was analyzed by macroscopic examination, chemical composition analysis, metallographic examination and hardness test. The results show that the chemical composition of the pipe was normal, and its property didn’t obviously deteriorate. The failure of the pipe was caused by the crack of weld of water-wall fin. The crack was intergranular reheat crack, which initiated at the coarse-grained heat-affected zone(CGHAZ) near the weld fusion line, propagated along heat-affected zone, and then extended to the base metal, eventually resulting in the leakage of the pipe. The microstructure of the coarse-grained heat-affected zone was coarse martensite and bainite with obvious fragility, and the hardness of CGHAZ was obviously larger than that of weld zone. There is obvious the difference in microstructure and hardness between CGHAZ and weld zone, so stress concentration easily occurred at CGHAZ near the fusion line. As a result, intergranular cracking took place during high temperature operation.