Abstract: Multi-line lasers were applied to the on-line defect detection for steel rails instead of single-line laser,which solved the problem of "pseudo defects" due to the vibration of rails under the mode of single-line laser. Images of multi-line lasers captured by camera were processed to realize the online detection and the accurate defect feature area was acquired with steps of the extraction of strip centers,detection of difference algorithm,discrimination between real and pseudo defects,image mosaic,etc. Images of the defect area were extracted by the cross-correlation and self-correlation information of laser curves,and there was no need for the step of depth mapping under the single-line mode,so the problem of "fault detection" caused by the vibration of rails was solved fundamentally. The technique is applied to on-line detection of defects for the bottom side of steel rails,and the detection rate is much higher than manual detection.