Abstract: A cracking phenomenon occurred in the corner weld of the high-temperature superheater header of a gas turbine waste heat boiler. The reasons for the cracking of the weld were analyzed by metallographic examination, mechanical property testing and other methods. The results show that poor welding quality, high weld hardness, and abnormal microstructure were the intrinsic causes of weld cracking in fillet welds.The alternating stress, structural stress, and thermal stress caused by the smoke baffle were the external reasons for weld cracking.