In order to detect the internal defects of the optics effectively, the scattering images of the defects are obtained after multiple total internal reflections of the laser beam in optics using total internal reflection technique. The three-dimensional position information of the defect is obtained by image processing technique, such as ellipse fitting based on least squares method. The proposed method is verified experimentally, and the experimental results show that 35 scans can complete defect detection of large optics with size of 150 mm×120 mm×20 mm. The positioning accuracy of the defect depth for samples to be tested is smaller than 150 μm. It indicates that the method can detect the defects of large optics effectively. Furthermore, the error sources that influence experimental result and factors that limit the system resolution are analyzed. The results show that the system resolution can be effectively increased by increasing the lateral resolution of the imaging system or reducing the cross-section width of laser beam.