In order to use laser ultrasonic technology to detect the surface crack depth of the sample, a laser ultrasonic non-destructive testing model was established based on the thermal-solid coupled physical field of the finite element analysis software (COMSOL), and cracks of different depths were simulated. Based on this, the quantitative calculation formula of crack depth was deduced according to the propagation law of ultrasonic waves in metal and the crack depth was calculated by experiments. The calculation formula was highly accurate. The results show that as the crack depth increases, the reflected shear wave SR at the bottom of the crack also increases, and the two have a linear relationship. The quantitative formula for crack depth proposed in this paper has higher accuracy, and the relative quantitative error of crack depth does not exceed 7%.