Abstract: The structure characteristics of the three types of thermocouple fillet weld(saddle-type, semi and full plug-in) of coal-fired power plant were analyzed. For the whole testing for fillet weld, probes with two K values were used. Through the practical testing, combined with the calculation formula, probe K value for each type of fillet weld was selected, the reflected waves caused by the hole of plug-weld were excluded, the defect depth was determined by reflected wave. Testing results well meet with the actual autopsy results, and it showed that ultrasonic testing for the fillet weld of coal-fired power plant was possible and practicable.