To verify the applicability of 25 m cast-in-place steel spring floating slab track in urban express rail transit， the vehicle-track coupled dynamics model and numerical method of CRH6 and cast-in-place steel spring floating slab are established based on the vehicle-track coupled dynamics theory. Through simulation calculation， the coupled dynamics performance of cast-in-place floating slab track bed and train under fast driving conditions is simulated， -and the safety， comfort and stability of 25 m cast-in-place steel spring floating slab track bed are evaluated. The effect of cast-in-place floating slab length， thickness and support damping on the dynamic performance of the system is studied. The research results show that： The denser the vibration isolators are， the better the stability of the track bed itself； The variation of the thickness of the cast-in-place steel spring floating slab bed on the operational safety indicators and ride comfort indicators of the intercity electric multiple units are not large； increasing the thickness of the floating slab can slightly improve the running stability of the vehicle； When the intercity electric multiple units is running， the stability indicators， the comfort rate and other indicators all meet the limit， that is， the cast-in-place floating slab track can meet the requirements of urban express rail transit. The research results can provide support for the dynamic design of the cast-in-place steel spring floating slab track with a speed of 160 km and above.