With the rapid development of industrial technology, the requirements for the imaging accuracy of defect detection are increasing day by day. To achieve precise imaging and positioning of tiny internal defects in the pipe wall, a method combining phased array laser ultrasonic and frequency-domain synthetic aperture focusing technology for defect imaging was adopted. A defect imaging optimization method for laser ultrasonic signal preprocessing was proposed to achieve rapid and high-quality detection of internal defects. Firstly, the excitation and propagation process of ultrasonic transverse waves under the condition of phased array laser source excitation was simulated by the finite element method. The ultrasonic acoustic field interacting with the defect was analyzed and the scanning signals were collected. Then, the signals were processed by differential calculation, envelope extraction and time-domain windowing. Finally, the phase shift migration method was used to achieve acoustic field inversion. Compared with the imaging results without preprocessing, it is shown that the proposed optimization method reduces the average positioning error of the defect in the transverse and longitudinal directions by approximately 20% and 3% respectively. The signal-to-noise ratio of defect imaging is improved by approximately 45%, and the efficiency is increased by approximately 50 times compared with time-domain imaging. The proposed frequency-domain defect imaging optimization method realizes precise positioning and high-quality imaging of defects, providing a feasible solution for real-time imaging in laser ultrasonic non-destructive testing.