It is known to all that air heater plays an important role in the ground test of scramjet,and the numerical simulation has been a rather effective method to investigate air heater.The effects of different types of air injection region and mixing chamber towards the mixing performance are compared in this paper,the investigation results show the conclusions as follows: the different arrangements and structures of mixing holes of baffles affect the uniformity of the outlet,it is proved that the design condition is rational ; there exit recirculation zones near the wall because of the high speed in inlet of mixing chamber,the effect of recirculation zones on mixing effect is great,so the application of fence will be researched in future to improve mixing efficiency,furthermore,the velocity distribution in axial cross sections is rather nonuniform; finally,it is feasible to shorten properly the length of mixing chamber,whereas shortening excessive will reduce the uniformity of the outlet and combustion efficiency of air heater.The results provide a reliable criterion for the configuration design and improvement of air heater.