This study uses a three-dimensional, compressible, turbulence model to investigate the alleviation effect on tunnel aerodynamics of equal-transect ring oblique tunnel portals with different slope values. The turbulent flow around the train body is computed using the RNG κ-ε turbulence model, and a sliding mesh method is utilized to treat the relative motion of train and tunnel. The numerical results are verified through the results of moving model experiments. The mitigation effects of ring oblique tunnel portal on the initial compression wave are analyzed. The relationship of micro-pressure wave and slope values is proposed, by which the micro-pressure wave induced by high-speed train entering tunnel with equal-transect ring oblique tunnel portal can be estimated rapidly. The accuracy of the proposed relationship is validated by previous studies and moving model experimental data. The estimation results using the proposed formula are in good agreement with data from references and moving model experimental tests. Results also show that the maximum pressure gradient and micro-pressure wave can be reduced by about 10.8% when slope value is 1:1.75 relative to slope value of 1:0.5.