Abstract: Corrosion under the insulation layer is a kind of damage that often occurs and is difficult to be avoided in pressure pipeline. The detection accuracy of pulse eddy current and ultrasonic guided wave is low, which can not meet the requirements of regular and accurate thickness measurement of pipeline. The digital ray boundary method is used to measure the wall thickness of insulated pipeline, and the measurement error is analyzed theoretically. The results show that the digital ray boundary method has high detection accuracy and can be used in engineering practice.