The stable operation of mechanical parts is the top priority to ensure the safety of production. Internal defects are an important factor causing the failure of mechanical parts. However, it is difficult to locate and quantify the internal defects of mechanical parts at present. Therefore, this paper realized the quantitative characterization of internal defect parameters, and proposed a method to characterize the Angle of internal defect. In this paper, based on the principle of laser ultrasound, the influence of internal defects on ultrasonic body wave time domain signals is analyzed through numerical simulation combined with experimental methods, and the location, depth, length and Angle of the quantitative characterization of the main internal defects are studied. Experimental results show that the pulse reflection method can accurate characterization of the internal defect parameters, swept through A experiment on the depth of internal defect characterization, the relative error within 5%, the length of the internal defect characterization by B scan experiment, the relative error is within 4%, through the way of combining A flicking A and B, realizes the quantitative characterization of Angle of internal defects. The results provide a reference for the quantitative characterization of internal defects of mechanical parts.