Abstract: The cracking cause of TP347H steel reheater tube of circulating fluidized bed boilers in a power plant was analyzed by macroscopic morphology and microstructure observation, chemical composition analysis, hardness measurement and impact property test. The results show that after TP347H steel reheater tube was welded with fins, the metallographic uniformity near the weld bond decreased, and the grains in local area coarsened, resulting in the poor mechanical properties. During the service process, the crack initiated at the grain boundaries near the weld bond of the reheater tube under the combined action of the welding residual stress and the additional stress generated by the platen reheater tube vibration. The oxidation reaction during high temperature service accelerated the cracking of the reheater tube. It was recommended to replace the TP347H steel with fine grain materials and optimize the structural arrangement of the reheater to prevent the reheater tube from cracking.