In this paper,a characterization parameter based on the sum of laser ultrasonic broadband transmission coefficient was proposed to realize the quantitative detection of surface crack depth. By means of finite element simulation and experimental measurement,the interaction mechanisms between Rayleigh waves generated by laser and surface crack are analyzed in time domain and frequency domain. The sum of wideband transmission coefficient is adopted as the physical parameters of crack depth,and established the fitting equations with the surface defect depth. The results show that the Rayleigh wave generated by pulsed laser is very sensitive to the crack depth of metal surface,and the proposed method can be used to accurately determine the crack depth of metal surface. The research results of this paper can provide potential applications for noncontact detection of surface cracks on workpiece.