Due to the limitation of traditional methods for the depth detection of material surface defects, a laser pulsed thermography technique in transmission mode is proposed in this paper. A laser source is selected as the excitation source to heat the defect surface of the measured material. The excitation point is selected directly below the surface defect. The laser power is selected as 50 W, and the heating time is selected as 1 s. During the heating process, the temperature field on the back of the material produces temperature difference due to the influence of heat flow in the conduction process of surface defects. Infrared thermal imaging camera records the temperature field changes on the back of the material, and point A without defect is used as a reference point, and point B at a defect is used as an experimental point. By analyzing the temperature changes at points A and B, the depth of surface defect is extracted. Experimental verification shows that this method can detect the depth of defects on the material surface under certain conditions. When the temperature of point A is constant, the best fitting relationship between the temperature of point B and the depth of the defect becomes an exponential relationship. The temperature of point B on the back also decreases. The research results lay a foundation for the subsequent accurate quantification of defect depth.