Abstract: Grain size of base metal and weld of thin-walled 304L austenitic stainless steel welded tube after solid solution treatment at different temperatures (800-1 350℃) were counted by tests. The ultrasonic signals of base metal and weld were triple wavelet decomposed by ultrasonic attenuation method based on continuous wavelet transform to extract the characteristic energy. The feasibility of ultrasonic attenuation method to evaluate grain size of welded tube was studied. The results show that with increasing solid solution temperature, the base metal grain size and the corresponding ultrasonic characteristic energy decreased; in the actual detection, the relative characteristic energy threshold of ultrasonic signals could be set between 600 and 1 000; in this case, the grain size was between 7 and 8.5, and the grain size was qualified. With increasing solid solution temperature, the grain size of weld increased, and characteristic energy first increased and then decreased and then increased, and reached the maximum at about 1 050℃; in the actual detection, the relative characteristic energy threshold of ultrasonic signals could be set between 140 and 180; in this case, the grain size was 8.5-9, and the grain size was qualified. Ultrasonic attenuation method could be used to quickly and nondestructively evaluate the grain size of thin-walled 304L austenitic stainless steel welded tube.