Abstract: In order to solve the problems of the on-line nondestructive testing technique, the paper carried out the researches on the on-line inspection. On-line inspection instruments and methods applied to industrial pipes under 250 ℃ were introduced to fill a gap in the conventional testing methods. This paper reported a magnetic particle testing method of ferromagnetic materials in high-temperature. The inspection sensitivity with the method is not lower than that at normal temperature. The measurement method of transverse wave velocity in the carbon steel used in pressure pipeline system in high temperature was established, and the change pattern of transverse wave velocity in the carbon steel in high temperature was investigated. The paper also analyzed the correction method of ultrasonic testing defects(reflectors) in high-temperature, and solved some ultrasonic transverse wave technical problems of weld defect under the high-temperature environment, which were urgent to be solved in actual engineering.