A dynamic experiment for CRTSⅡ slab ballastless track - bridge system under the train running condition was carried out. The dynamic response of CRTSⅡ and standard 32m simply supported prestressed concrete box girders was measured under CRH380A-001 trains traveling at the speed of 285 to 350 km/h. Through the field collection and data analysis, the vertical and transverse acceleration amplitudes of the rail, track plate, base plate, and beam panel were obtained. The vertical and transverse absolute displacements of the pier top were also obtained. The results show that abrupt wave crests appear, as the acceleration of each structure increases fast and then reduces significantly when the train speed reaches about 295 km/h. At that time, the vehicle induced acceleration response of the rail-track plate-base plate-beam panel has a significant decline trend, and the vibration attenuation is relatively obvious. Furthermore, the simplified method for computing the dynamic deflection of a girder was analyzed based on the measured natural frequency and damping ratio of the girder, and the calculated values are close to the measured ones. The study results can provide the basis for improving the numerical analysis model and verifying the calculation results.