For the measurement of different crack depth ranges on metal surface, the study is carried out by extracting the characteristic points of surface acoustic wave reflection component. Using COMSOL Multiphysics to perform finite element analysis on the process of laser ultrasound acting on the surface of aluminum plate, the obtained surface acoustic wave in the time domain signal is bipolar. When the surface acoustic wave propagates to the surface defect, the reflection echo RR1, RR2, and the mode conversion wave RS are formed, which appear as an obvious oscillation signal in the time domain signal. Based on the signal, the characteristic points that conform to the characteristics of the reflected component are obtained, and the arrival time of the reflected echo is obtained. The position of the defect front and the depth of the defect are calculated. The study finds that the location of surface cracks can be obtained by the arrival time of RR1. When the crack depth is small, which is less than or equal to the depth range of the surface wave penetration at the crack front (ranging from 1 to 3 surface wave lengths), the crack depth can be calculated by the arrival time of RR2. When the crack depth is larger than the depth of surface wave penetration at the crack front (more than 3 surface wave lengths), the crack depth can be calculated by the arrival time of RS. Finally, according to the difference of the crack depth range, a piecewise function for calculating the crack depth is obtained, which provides a reference method for detecting the crack depth on the metal surface in practical applications.