In order to evaluate surface breaking cracks, laser ultrasonic diffraction shear wave method was used to measure its parameters. Laser-induced shear wave propagating to the crack tip produced a diffraction signal and then the signal was received by the sensors on the other side of the crack. According to the transit time of diffracted shear waves at different receiving points, the formulas for calculating crack parameters were derived. The propagation process of the shear wave in the workpiece was simulated and analyzed. The experimental system of laser-ultrasonic crack measurement was established. The diffraction shear wave data were also collected. The experimental results show that, within the effective detection range, the error between the measured value and the actual value is within 5%. The results can promote the application of laser ultrasound in surface crack detection.