In order to obtain the relative optimal opening rate of the open-cut tunnel portal, the three-carriage high-speed train running through the hood with different opening ratios (20%-84%) at different speeds (250, 300 and 350 km/h) was modeled by using sliding mesh method, and the three-dimensional unsteady compressible Reynolds-averaged N-S equation and standard κ-ε equation turbulence formula were employed in this numerical model. Then the initial compression wave and micro pressure wave were simulated and analyzed. The results indicate that the numerical results show a well agreement with the dynamic model test, and the deviation between them is no more than 6%. The amplitude of initial compression wave increases when the hood with holes, and it increases at first stage and then decreases with the increasing of opening ratios, and reaches the maximum value when the opening ratios is 60%. The pressure gradient of initial compression wave is significantly affected by the opening rate. It decreases with increasing of the opening rate, and the amplitude shows a small change when opening rate is greater than 40%. Similarly, the effects of opening rate micro pressure wave show the same rules as the pressure gradient.