A 3D CFD numerical model of tunneltrain-air was built to analyze the transient change characteristics of the aerodynamic loads of each carriage when a HST enters tunnel and the corresponding change of the running postures of each carriage，considering the unsteady compressible turbulent characteristics of the flow field when a highspeed train ( HST) enters into tunnel． The corresponding change mechanisms were uncovered from the perspective of flow field． The impact effect of aerodynamic loads on the carriage was discussed． The main conclusions are as follows: ( 1) The five aerodynamic loads of each carriage show remarkable sudden change characteristics，and accordingly，all carriages show instantaneous deflection and‘nodding’and other behaviors． ( 2) The great difference in longitudinal pressure difference between the two sides of the carriage is the fundamental reason of the sudden unloading of side force and rolling moment and the sudden increase of yawing moment． ( 3) Crosswind is the main factor that causes the impact strength of aerodynamic loads on the carriage to rise remarkably． ( 4) The safety coefficient of the head carriage is the key to control the traffic safety of the whole train．