Based on the vehicle-track coupled dynamics theory, the vertical vehicle-track coupled dynamic model is set up. Take a Chinese speed-up passenger car and speed-up track as an example. From the aspects of power spectrum density (PSD) and coherence function, the frequency domain characteristics of the vertical vehicle-track dynamic responses and the correlation between the carbody's vertical acceleration and the vertical irregularity are analyzed. Results show that: the main frequency of the carbody's vertical acceleration is about 1.63Hz; A peak appears around 1.6Hz on the vertical wheel-rail force spectrum, which is caused by the low-frequency vibration of the carbody. The coherence analysis found that the carbody's vertical acceleration and the vertical track irregularity have relatively strong correlation around 1.68Hz, which is very close to the main frequency of the carbody's vertical acceleration.