In order to study the flexible dynamic vibration characteristics of railway freight cars, a rigid-flexible coupling dynamic model of railway freight cars C80 was established by joint simulation of NASTRAN and SIMPACK. The model was verified by modal test and dynamic calculation, in which the maximum error of mode is 9.70%, and the maximum error of derailment coefficient and wheel load reduction rate is 6.64% and 8.15% respectively, which verifies the correctness of the rigid-flexible coupling model. Furthermore, based on the dynamic mechanical model, the dynamic performance, body vibration characteristics and magnification factors of key components of empty and heavy vehicles at different speeds are studied. The results show that the derailment coefficient of heavy vehicles is lower than that of empty vehicles, while the wheel load reduction rate and capsizing coefficient of heavy vehicles at higher speeds (100 km/h) are higher than those of empty vehicles. The vertical vibration characteristics between heavy vehicles and empty vehicles exhibit markedly distinct patterns. As operational velocity increases, the resonant frequency of empty vehicles demonstrates a monotonic decreasing trend, whereas the heavy vehicle body manifests a non-monotonic frequency response characterized by an initial increase followed by subsequent attenuation in resonant frequency. That of empty vehicles has two resonance regions in the direction of lateral vibration. The resonance region of the heavy vehicle is consistent with the second resonance zone of the empty vehicle, so it is necessary to pay attention to the influence of the low frequency vibration of the empty vehicle. The variation law of vertical vibration amplification factor of empty and heavy vehicles is different at different speeds. For empty vehicles, the variation of vertical vibration amplification factor decreases significantly with speed (the minimum relative 60km/h decreases by 56.3%), indicating that the vertical damping performance of empty vehicles increases with the increase of speed, while the amplification factors of side frame and car body increase with the increase of speed, among which the side frame changes more obviously at high speed. Compared with 120km/h, 140km/h increases by 162%, indicating that the vibration reduction performance of heavy vehicles is reduced when running at high speed.