Considering the three dimensional unsteady compressible turbulent characteristics of the flow field when a high-speed train (HST) enters the tunnel, a refined 3D CFD numerical model of tunnel-train-air was built to analyze the transient flow field structure and pressure distribution in the process of HST entering into tunnel under the conditions of crosswind and without crosswind comparatively. The transient change characteristics of the five aerodynamic loads (i.e. aerodynamic side force, lift force, rolling moment, yawing moment and nodding moment) of the train body under crosswind and the influence of wind and vehicle speeds on its maximum variation amplitudes were studied. The main results can be summarized as follow: When a HST enters into tunnel under crosswind environment, flow field structure and pressure distribution on both sides of the train body outside the tunnel have an obvious dissimilarity, and it's smaller inside the tunnel, which causes the sudden change of the train body pressure difference before and after the HST entering the tunnel. In the process of HST entering the tunnel, there are transient change effects in various aerodynamic loads of the train body, and the tail carriage presents capsizing，“jumping”，“snake-like” swing and “nodding” behavior at the same time. The change of the wind speed has a significant effect on the variation amplitude of the tail carriage yawing moment, while the change of the train speed has a significant effect on the variation amplitude of the head carriage yawing moment.