High‑speed inlet capture mass flow decreases when the upstream boundary layer thickens， which limits combined‑cycle propulsion performance. Focusing on the upstream part of the inlet of the integrated flight engine aircraft， numerical simulation methods were used to study the vortex dynamics mechanism of the Vortex Generator （VG） regulating the cross-sectional velocity density distribution. An opposed VG structure was proposed with the aim of maximizing the inlet capture mass flow rate. The results indicate that when the incoming Mach number Ma=8， the VG can increase the inlet capture mass flow by 173%. VG achieves flow control by inducing a pair of vortices with opposite directions of rotation. On the up-wash side of the streamwise vortex， low-energy fluid in the boundary layer is swept into the mainstream， resulting in a loss of velocity profile and a decrease in velocity density； on the down-wash side， high-energy mainstream is transported to the boundary layer， with a full velocity profile， thinning of the boundary layer， and rising of velocity density. If the down-wash effect is stronger than the up-wash effect， the overall performance of the streamwise vortex is a gain in mass flow capture. When the streamwise vortex is tangent to the down-wash side， its interaction will strengthen the down-wash flow， enhance the transport of high-energy mainstream to the boundary layer， and improve mass flow capture. On the contrary， if the up-wash side is tangent， the up-wash effect is enhanced， which is not conducive to mass flow capture. Opposed VG enhances the ability to transport mainstream to the boundary layer by strengthening the down-wash effect of streamwise vortices， thereby increasing downstream inlet mass flow capture.