A large number of hot cracks in the fillet welds of SA-213TP347HFG membrane water wall which were assembled as high temperature super heater and reheater parts produced by a boiler manufacturing company. This paper describes the characteristics of crack morphology and fracture morphology through macro observation and microstructure observation. The causes of thermal crack formation are analyzed from the aspects of flat steel surface quality, welding material selection, welding heat input and restraint stress, energy spectrum analysis and so on. It is found that the poor surface cleanliness of fins and choosing ER309LSi as welding material are the main causes of a large number of hot cracks in the fillet weld of SA-213TP347HFG membrane water wall, providing a theoretical basis for the treatment of similar defects.