In order to detect and locate crack defects in aluminum plate, in this paper, COMSOL is used to establish the internal crack defect model of aluminum plate and analyze the action mechanism between laser ultrasonic and defects with different buried depths. Compared with transverse waves (S waves) and longitudinal waves (L waves), surface waves (R waves) have obvious interaction with near-surface defects with a buried depth of less than 1 mm. Therefore, R wave is also more suitable for detection in the buried depth range, whereas S wave and L wave are more suitable for detecting internal defects with buried depth greater than 1 mm. This paper proposes methods for optimizing positioning and quantitative length measurement. The amplitude and peak position of transmitted surface wave (TR) can be used to quantify the buried depth and length of near-surface defects. The accuracy of crack length quantification can be improved by using reflected surface waves (RR) to assist the quantitative calculation of crack length. In addition, according to the analysis of the acoustic wave path, this paper proposes that the arrival time of the reflected wave (2S) and the creeping wave along the crack (SCS) at the crack can be used to quantitatively estimate the buried depth of cracks with greater than 1mm, and the feasibility was verified by simulation. The method proposed in this paper lays a theoretical foundation for real-time detection of defects in additive workpieces.