Abstract The heterojunction structure of CoFeB/heavy metal has shown significant potential for spintronics, where both electrons and magnons potentially can serve as information carriers. However, another promising information carrier, coherent phonons, has not been fully explored for hybrid phononic and spintronic devices. In this study, we used time-resolved pump-probe spectroscopy to investigate the dynamic behavior of coherent phonons. We observed variations in reflectivity spectra, corresponding to changes in phonon frequency and relaxation times, with different thicknesses of the heavy metal and CoFeB layers. The experimental results demonstrated a decrease in coherent phonon oscillation frequency as the thickness of the CoFeB and heavy metal layers increased. These findings were further supported by first-principles calculations, which showed that the frequency of the optical modes is suppressed due to interface relaxation between the magnetic and heavy metal layers.