To deal with the detection of the internal defect in the thick steel plate, we adopt the synthetic aperture focusing technique (SAFT) to locate and image the internal defect of the thick steel plate. The moving pulse laser line source is used to generate the longitudinal ultrasonic waves in the steel sample. The laser vibrometer is applied to obtain the time-domain ultrasound B-scan signals at the fixed point. The longitudinal echo-waves reflected by the defect are extracted from the time-domain signal. And the imaging of the internal defect of the sample is realized. This imaging process is simulated numerically by the finite element method (FEM) and verified in the experiment. The experimental results coincide well with the numerical calculation results. The proposed defect imaging method can realize the defect detection under the circumstance where the defect echo-waves have a low signal-to-noise ratio (SNR). The process of this method is simple and convenient, and the method has practical value in the laser-ultrasonic detection field.