Abstract: Taking frequent cracking leakage accident of water wall tube in a 600 MW supercritical W flame boiler early producted by Dongfang boiler factory as example, the macroscopic analysis, temperature field and stress analysis, operation parameters analysis, chemical analysis, metallographic analysis and mechanical property analysis of the water wall tube were carried out. The results show that based on the uneven distribution of heat load in furnace structure of W type flame boiler and welding residual stress, there are problems in the structure and designed dimensions between fin and water wall. Because of large width of the fin, large temperature gradient of the fin, the internal stress of the fin is too large, and under the joint action of the alternating stress during start and stop of the unit and the variable load, the fatigue cracks appear and propagate on the fin, which eventually leads to tensile cracking of the tube.