In order to realize the quantitative detection of surface V-shaped cracks, this paper uses finite element method to study the mechanism of laser-excited ultrasonic surface waves and surface V-shaped cracks. The characteristics related to the length and angle of surface cracks are extracted from the reflected characteristic signal. The influence of the angle and length of the crack on the surface wave signal is analyzed, and the time-frequency analysis of the time-domain signal is carried out, and then a method to quantitatively characterize the length of the surface V-shaped crack using reflected surface wave is proposed. The numerical results show that with the increase of the crack length, the time difference of the reflected echoes also linearly increases, and the transmitted surface wave is more sensitive to the change of the crack angle. Through the relationship between the time difference of the reflected echo arrival and the crack length, the crack length can be quickly judged, and the length of the corresponding crack can be quantitatively characterized. The numerical results are in good agreement with the theoretical results, which provides an efficient and accurate method for the quantitative detection of the length of surface V-shaped cracks.