Economizer tube leakage occurs after 34 days of use,and the leakage point is at the butt weld of the steel tube. In order to find out the leakage reasons,the base material composition test,microstructure observation and hardness test of the weld section are carried out,and the test results are compared with the specified requirements in the standard ,the material composition and hardness test results meet the requirements of relative standards. By observing the microstructure,it is found that the pores and internal cracks in weld fusion zone cause the decrease of the weld strength.In use process,the extension of internal cracks in work environment is the main reason for steel tube leakage at weld.