Laser ultrasonic detection technology has the characteristics of non-contact, wide bandwidth and high resolution. The conventional ultrasonic is not sensitive to the small defects of nuclear damage inside the rail head, which is difficult to locate and quantitatively detect. In this study, the calculation model of the defect location quantitative detection based on the flight time of the modal conversion reflected wave and diffracted wave is proposed, which is based on the principles of laser ultrasonic body wave scattering and diffraction. By using COMSOL simulation, a two-dimensional finite element model of the interaction between the ultrasonic body wave and the internal defect is formulated. The model conversion state of the ultrasonic body wave at the internal defect of the rail is analyzed, and the feasibility of the positioning and quantitative method is verified. Secondly, a fixed scanning laser ultrasonic experiment detection system is established to implement B-scan experiments on hole defects of different buried depths and diameters. Experimental results show that the laser ultrasonic detection technology can effectively detect the micro hole defects in the rail. Based on the calculation model and the proposed detection method, the positioning relative error of the detection of the internal defects of the rail is within 6% , and the quantitative relative error is within 9% .