The high-precision distributed acquisition of high-speed rail seismic waves were achieved using the High-Definition Distributed Acoustic Sensing (HD-DAS) system. The sensor unit used simple communication optical cable to achieve good monitoring effects at low cost, and could be easily deployed according to actual environmental conditions, greatly improving the efficiency of field observation work. By using multiple parallel optical cables, noise could be effectively suppressed and the signal-to-noise ratio of high-speed rail seismic signals can be improved. Data analysis and processing were performed on seismic signals collected by HD-DAS from high-speed rail. The analysis revealed that the signals display discrete spectral characteristics. Further analysis was conducted on the attenuation law of seismic wave signals, and the transmission speed of seismic waves near the surface was obtained through the frequency-wavenumber spectrum. This indicated that the high-speed rail seismic wave signals acquired through optical fiber could be used to detect underground structures and monitor changes in the medium’s state, thereby enhancing the safety of high-speed rail operations.