Abstract: Ballastless track is a typical layered concrete structure, and the delamination defect is its most common form of damage, which affects the safe operation of high-speed trains. The traditional synthetic aperture focusing imaging (SAFT) method is an ultrasonic testing method based on constant sound velocity. Ignoring the differences between the interlayer acoustic impedance and the refraction of the acoustic wave at the interlayer interface makes it difficult for the acoustic beam to focus at the defect, and the temporal error of the acoustic wave propagation in the layered structure is large. Therefore, a synthetic aperture focusing imaging method for multilayer structures is proposed in this paper, which fully considers the difference of sound velocity between layers in multilayer structures and uses ray tracing method to accurately obtain the propagation times of sound waves in multilayer structures. On this basis, the effects of different incident wave patterns and different excitation frequencies on the results of synthetic aperture focusing imaging of multilayer structures are analyzed. The research results show that with the multi-layer structure synthetic aperture focusing imaging method, the imaging resolution of shear wave incidence at 50 kHz is higher, and the detection effect of the delamination defects of ballastless track is better, which provides a theoretical support for the detection of such defects.