ing at addressing the problem in detecting the internal defects of large thickness materials, the locations and sizes of internal defects are detected through the diffraction effect of defects on laser ultrasonic. First, the Comsol software is used to study the interaction process between laser ultrasonic and internal defects and we establish the calculation model for received laser ultrasonic signals. Using the laser ultrasonic testing system, an experiment on detecting the locations and sizes of defects in a 45-steel block are conducted. The experimental results show that the relative position and size errors in defect detection are within 5% and 10%, respectively. This experiment proves the feasibility of detecting the locations and sizes of internal defects using the laser ultrasonic diffraction bulk wave method. Furthermore, it achieves a non-contact detection via laser excitation and reception.