Abstract: As one of the three main units of coal-fired power plant, the stable operation of the boiler affects the safety of the whole coal-fired power plant directly, and the safety of the thermal pipeline is the most important, where accidents occur frequently. This paper introducesforming mechanism and harm of the three kinds of the defects which are common in the thermal pipeline: water wall sulfur corrosion, superheater and reheater corrosion, economizeroxygen corrosion. Also introduced in the paper are the technique principle, research results,advantages and disadvantages of the detection technology which can be used to detect these defects emphatically like the far field eddy current testing technology, leakage magnetic detection technology and the electromagnetic ultrasonic testing.