Abstract: For the tube-wall brazed joints in a 304 stainless steel heat exchanger with a finned-tube structure which experienced cracking and water leakage during service, samples taken from the cracked region were thoroughly analyzed to identify the failure mechanism of brazed joints of 304 stainless steel heat exchanger tubes using nickel-based brazing filler metals. Through penetration test, metallographic examination and scanning electron microscopy（SEM） observation, it is revealed that the non-uniform distribution of brittle phosphorous compounds reduces local toughness. Under operational stress, the joint undergoes intergranular fracture, generating microcracks that serve as whick origins for further crack propagation, ultimately leading to ductile fracture. Adding a small amount of copper to the brazing filler promotes the formation of a ductile nickel-based solid solution within the joint, thereby reduces the formation of brittle intermetallic compounds, and enhances the strength of the brazed joint in certain extent, which effectively avoids the formation of microcrack and improves the service reliability of the heat exchanger tubes. These findings provide theoretical support for improving the quality of brazed joints in 304 stainless steel.