The micropressure wave radiated from a tunnel exit is one of the environmental problems which can be investigated from the temporal pressure gradient of the compression wave. The effects of inclined portals on the initial compression wave, specifically the maximum temporal pressure gradient, are numerically studied by solving the flow field during a high-speed train nose entering a tunnel, using the unsteady three-dimensional (3D) Euler equations. After mesh independency and temporal sensitivity tests of the numerical method, validations are conducted by comparing the numerical results with experimental and numerical data. The temporal gradients of pressure wavefront are parametrically investigated for different combinations among the train speed, the blockage ratio of the train to tunnel, and inclination angle of the tunnel entrance. The numerical results show a negligible influence of train Mach number or blockage ratio on the normalized pressure gradient and noticeable effects of inclination angle, location of the train with respect to the median line of a double-tracked tunnel (DT), and the profile of train nose. Based on the numerical results, an empirical formula is proposed to predict the relationship between the maximum pressure gradient and the inclination angle of tunnel entrance.