Currently, laser ultrasonic synthetic aperture focusing technology (SAFT) is being increasingly used for imaging and detecting internal defects. A method, based on the synthetic aperture postprocessing imaging methods, for achieving high-quality detection of rectangular defects inside a specimen is proposed using laser ultrasonic mode wave imaging optimization under multifusion preprocessing signal technology. Differential calculation, envelope extraction of signals, linear interpolation of maximum values, and removal of artifacts using the mean value are performed on the detected laser ultrasonic mode waves, and then we select the processed mode waves for SAFT imaging. The comparison with the imaging results from mode waves without prior processing indicates that the postprocessing imaging method can accurately and qualitatively identify defects. Furthermore, errors in longitudinal depth detection and transverse width detection of defects are reduced by 2.37% and 30.88%, respectively, improving the accuracy of defect size and their location determination. Thus, the imaging quality is considerably improved, as evidenced by a 1.88-fold increase in the average value of the energy gradient (EOG) function, which characterizes imaging quality. This postprocessing imaging method achieved qualitative and accurate identification of internal defect positions, precise size detection, and high-quality imaging, providing a promising postprocessing solution for laser ultrasound-based internal defect detection methods.