To study the impact of low speed ejection on aerodynamic heating of hypersonic aircrafts，the flat injecting of large area under hypersonic flow condition is numerical simulated in this paper. The influence of ejecting pore structure, incidence angle and injecting inlet velocity on boundary layer flow field are discussed. Wall heat flux under different ejecting pore structures variation of ejecting factors and flow parameters with inlet velocity are obtained. The results show that low speed gas ejecting can relieve the aerodynamic heating situation of injecting regional wall and the downstream wall to some extent. The bigger the inlet velocity, the better the thermal reduction will be. As for four ejecting pore structures the wall heat flux is the lowest in ejecting pore structure 4 (surface ejecting pore structure), the other three ejecting cooling effect are basically quite. The thermal reduction of angle of attack α= 10° is obviously better than the state of α=0°. When injecting inlet velocity is 20 m/s, the injecting factor of injecting area is about 0.23, which means the averaged wall heat flux decreases about 23% under α=0°. And the injecting factor of injecting area is about 0.45 which is about double of the value of α=0°.