The carbody shaking phenomenon (CSP) can greatly reduce the ride comfort of high-speed trains. To investigate the abnormal vibration feature and causes of the CSP, extensive field measurements and numerical simulations were conducted. A three-dimensional rigid-flexible coupled dynamic model of a high-speed vehicle is constructed to reproduce the CSP and reveal the key influence factors, in which the finite element model of the carbody is updated based on the experimental modal data. The field test show that the main vibration frequency of the carbody accelerations appear around 7.90 Hz, which is close to the natural frequency of the carbody diamond mode. The simulation results demonstrate that the CSP is caused by the carbody resonance vibrations induced by bogie hunting motion, which is triggered by the deterioration of wheel-rail contact geometry. When the train passes through turnouts at high speed, wheel-rail impact will further aggravate the CSPs.