As the train speed increases, there are two typical dynamic response patterns of subgrade, i.e. with or without wave propagation. Once the wave propagation occurs, the track displacement begins to increase sharply and the environmental vibration becomes more and more significant. The determination of the critical speed at where waves are emitted is significant for relative investigation and engineering practice. To this end, the dynamic behaviors of a track-embankment-ground system in speed-frequency domain were discussed and a determination approach of the critical speed was proposed. To confirm the prediction ability, verifications were carried out on typical semi-infinite ground and track-embankment-pile reinforcement ground system. The results indicate that the phase plot performs satisfactory in predicting the critical speed of subgrade with arbitrary section. The appearance of phase shift reflects the intrinsic feature of energy dissipation with radiation of waves.