Although horizontal well technology has been widely adopted in hydrocarbon exploration, acoustic logging interpretation remains challenging under complex formation conditions, particularly when formation interfaces are located adjacent to the borehole. This study systematically investigates acoustic wave propagation in horizontal boreholes through numerical modeling, focusing on formation interface effects. Numerical models with varying positions and numbers of interfaces were developed, and 3D finite-difference wavefield simulations were performed. Wave mode characteristic parameters (velocity, energy, and dominant frequency) of full-wave array data from these models were extracted to analyze the effects of formation interfaces. Results demonstrate that P-waves are more sensitive to formation interfaces than S-waves, exhibiting distinct variations in propagation velocity, energy enhancement, and frequency fluctuation. In contrast, S-waves maintain stable propagation velocities with only minor perturbations in energy and frequency. Wavefield snapshots clearly show that P-waves in the faster outer formation gradually overtake those in the slower inner formation, arriving earlier at receivers. Furthermore, closer interfaces induce earlier wave overlap, while increasing interface distance delays the overtaking of the slow P-wave by the fast P-wave, and sufficiently distant interfaces completely prevent the overtaking. The proposed theory remains valid for complex formations involving multiple coupled interfaces and small-dip tilted interfaces. These findings elucidate the underlying mechanism of formation interface effects on acoustic wave modes within horizontal wells, providing a theoretical basis for accurate parameter extraction and correction.