In order to obtain deformation characteristics of a simply supported beam bridge with CRTS II ballastless track under dynamic loads, based on field tests, structural deformations of the track-bridge system were measured during CRH380A-001 train passing through a 32m long simply supported beam bridge with longitudinal continuous plate type ballastless track at the speed of 240 to 350 km/h. Through signals collecting on site and data analysis, vertical and transverse absolute displacements, horizontal break angle and beam-end rotary angle of the beam bridge structure, vertical and transverse relative displacements of the track structure and relative displacements of the pier beam were obtained. The resonance speed and dynamic coefficient of the bridge were studied. The results showed that under dynamic loads, the minimum vertical ratio of span to deflection of the beam is 54000, the minimum horizontal one is 150000, they are far larger than the minimum limit values in the code; the maximum values of the rotary angle at the beam-end and the beam horizontal break angle are 0.077‰ and 0.119‰, respectively, they are smaller than the limit values in the code; furthermore, the simply supported beam bridge with CRTS II ballastless track has the second-order vertical resonance velocity of 306km/h and the third-order transverse resonance velocity of 312km/h obtained with actual measurements, they agree well with the theoretical resonance velocities of 309km/h and 315km/h, respectively; the dynamic coefficient actually measured near the resonance velocity is bigger than the limit value of 1.084 in the code, its maximum value reaches 1.18.