Abstract: The working principle and application of three kind of ultrasonic detectors of p ipeline are introduced.These ultrasonic detectors can be in-line used to detect the corrosion defect and its position (internal wall of pipe) by means of collec ting pipeline data like wall thickness and position of kilometer number along pi peline. However, some defect is existed in these ultrasonic detectors, their def ects are easy to see.take the piezoelectric and EM wave ultrasonic detection met hod for example, for the former, slow detection progress, complicated operation, and coupling agent is needed; for the latter, low sensitivity to corrosion defect s and poor capacity against electromagnetic interference. And more things are ne eded to do to improve their suitabilities. The paper makes prediction on applica tion and tendency of pipeline ultrasonic detectors.