Ordinary, the weld joint of 20# steel of the steam boiler to 316L stainless steel of the steam pipeline was made by argon arc welding method with the following welding parameters: a welding current of 80 A, an arc voltage of 11 V, and a welding speed ranging from 6 to 8 cm/min. In this way, the weld joints of 20#/20# steel and 20#/316L steel were welded with carbon steel and 316L steel as filler material respectively. However, even under normal operating conditions, instability fractures occur at the joints before the specified service time is reached. To investigate the corrosion fracture mechanism, samples of the above two types of joints are immersed in a simulated water to analyze the weight of environmental factors on the corrosion rate of the joints. Additionally, four-point bending tests are conducted to simulate the conditions experienced of the joints during service. Surface morphologies are observed using scanning electron microscopy, while the phase composition of corrosion products is analyzed via infrared spectroscopy and Raman spectrometer. The results indicate that the corrosion rates of 20#/316L steel welded joints are higher than those of 20#/20# steel welded joints across different environments, with temperature being the decisive factor influencing the corrosion rate. A significant amount of strip-like ferrite is present in the heat-affected zone on the 20# steel side of the 20#/316L steel welded joint, which is prone to initiating pitting corrosion. Under the combined effects of residual stress and structural stress caused by welding, severe stress concentration occurs at the bottom region of the prefabricated notch, inducing stress corrosion.