An active flow control strategy based on onboard blowing and suction was proposed to mitigate the aerodynamic effects induced by ultra-high-speed trains entering tunnels. A three-dimensional, compressible, and unsteady Reynolds-averaged Navier-Stokes (URANS) numerical model was established to analyze train-tunnel aerodynamic responses, including pressure waves and aerodynamic loads, under three control modes (front suction with rear blowing, front suction only, and rear blowing only) and different slit areas. Results demonstrate that the combined front suction and rear blowing mode significantly attenuates pressure fluctuations on both the train surface and tunnel wall, with an average peak-to-peak pressure reduction of 18%, outperforming the single modes. Specifically, front suction weakens the head compression wave and suppresses the positive pressure region ahead of the train, thereby effectively reducing the initial pressure gradient. Meanwhile, rear blowing alleviates the tail expansion wave and negative-pressure region, which helps improve flow stability. The peak-to-peak pressure control effect exhibits an approximately linear decay relationship with the blowing/suction slit area: when the slit area is 4 m², the maximum pressure difference peak-to-peak values on the train and tunnel are reduced by about 16.6% and 17.4%, respectively. The proposed strategy simultaneously weakens the compression wave generated at the train head and the expansion wave generated at the train tail, thereby reducing pressure fluctuations on the train surface and tunnel wall as well as aerodynamic loads on the train body. Moreover, increasing the slit area can further enhance the attenuation of pressure waves.