Abstract: Aiming to overcome the difficulty in detecting the internal defects of gauge angle of rail head by conventional ultrasonic, a method for quantitative detection of internal defects of gauge angle steel rail by ultrasonic longitudinal waves was proposed in this article. It established the inspection model of laser ultrasonic rail head internal defects through finite element simulation, analyzed the diffraction law between ultrasonic longitudinal wave and internal defects of different lengths in detail, and a quantitative detection theory was set up which realized the quantitative detection of the internal defects of the rail. The finite element numerical simulation results show that the laser ultrasonic longitudinal wave can effectively and quantitatively detect the buried depth and length of the internal defects of the rail head, and it can provide a reliable detection method for the quantitative detection of the internal defects of the rail.