In order to realize the quantitative characterization of pipe defects by laser ultrasonic technology, the interaction process of ultrasonic wave generated on the surface of metal pipe irradiated by laser and pipe defects was analyzed by finite element method. This paper explores the mode conversion of ultrasonic in the transmission process through the transmission image, and it also discusses the influence of defects with different height, width and angle on laser ultrasonic in the way of far-field excitation. Combined with the displacement signal of the receiving point, this paper analyzes the complex process of the interaction between ultrasonic and defects, and it analyzes the corresponding causes by applying Huygens principle. Finally, it puts forward a pipeline defect determination method Methods of quantitative characterization. The theoretical conclusion is verified by experimental research. The experimental research and theoretical conclusions show that the defect position can be quantitatively characterized by the receiving time of the reflected echo signal generated after the surface wave encounters the defect, the defect depth can be quantitatively characterized by the receiving time of the reflected surface wave signal generated at the bottom of the defect, and the defect angle can be quantitatively characterized by the defect angle. The quantitative characterization of the reception time of the bottom surface wave converted longitudinal wave signal lays a theoretical foundation for the reverse estimation of the size of the defect.