Abstract: The cracks in T23 steel water wall fin weld joints were analyzed by means of microscopic metallography, scanning electron microscopy（SEM）, energy dispersive spectroscopy（EDS） and so on. The crack specimen was taken from a USC boiler with almost 6 years service time. The crack initiates in the coarse grain zone of the heat affected zone of the joint, extends towards the inner wall of the tube, and finally ends in the fine grain zone. Further analysis on the cracks and their surrounding areas reveals that Ti（Al）-N compounds precipitated on the grain boundaries promote the formation of cavities, and then develop into wedge-shaped cracks and macro-cracks, which have characteristics of stress relaxation cracks. Analysis on the T23 fin weld service temperature field shows that the crack occurs between 380℃ to 520 ℃. Based on the above results, this paper draws the conclusion that the metal compounds will form cavities and cracks in the T23 steel weld-joints under the reheat crack sensitive lower limit temperature of 575℃, and puts forward the corresponding technical measures and suggestions.