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Nowadays X70 steel has been used as a core material for the construction of high-pressure, large-diameter and high-strength steel pipelines. The wall of piplines of an operating pipline system will deform in various degrees. Severe deformation will lead to microcracks or even a fracture. The ultrasonic flaw detection method is designed for the detection and evaluation of microcracks and defects caused by the deformation of X70 steel. However, an effective method to measure the stress level of X70 steel accurately is still lacking since the deformation of X70 steel often occurs within the elastic stage or plastic stage. Through conducting an experiment to study the tensile test of X70 steel specimens under a constant temperature, and measuring the propagation time and amplitude of ultrasonic LCR wave simultaneously to conclude the law of the propagation of ultrasonic LCR wave in elastic-plastic deformation of X70 steel, and determine the relationship between the propagation time and the amplitude of ultrasonic LCR wave and the deformation degree of X70 steel. According to the study, the deformation degree of X70 steel could be determined by the propagation time of ultrasonic LCR wave and the amplitude of the first wave: the level of stress could be calculated within the elastic stage and the deformation degree could be calculated within the plastic stage. The study results provide an effective support for the stress measurement of X70 steel by ultrasonic LCR wave. [ABSTRACT FROM AUTHOR]