Laser ultrasonic longitudinal wave sound field is distributed in all directions inside the workpiece.Compared to shear wave and surface wave,longitudinal wave reaches the opening crack first,followed by diffraction and wave mode conversion.If the corresponding ultrasonic signal which contains the crack information of the opening crack is extracted,the purpose of defect identification can be achieved.Based on the distribution characteristics of the ultrasonic longitudinal wave sound field generated by laser ablation,the time and phase characteristics of the diffracted longitudinal wave are analyzed,and the expression of the crack depth is given.A two-dimensional finite element model for the sample with an open crack is established,and the depth of the open crack is calculated based on the time of flight of the diffractive longitudinal wave.At the same time,the phenomenon of phase inversion of diffractive longitudinal wave is discussed,which can provide a basis for the identification of diffractive longitudinal wave during the detection process.Finally,a laser ultrasonic measurement experiment is performed;the test results show that it is feasible to use the diffraction signal to measure the open crack.