Abstract: This paper studies the natural gas(NG)-air mixing characteristics in engine intake and the effects of injection pressure, flow speed and injector alignment. The ejection-type wind tunnel was built as a test rig. The air mass flow rate was determined by sonic throat in the duct and schlieren photograph was used to visualize the mixing flow-field. The schlieren pictures were obtained at different injection pressure, sonic throat height and injector alignment. The results show that the NG-air mixing is independent of the throttle angle less than its critical value at the specified flow speed as well as the NG injection pressure. When the values of throat height are respectively 16.4mm and 7. 1mm, the corresponding values of critical throttle angle with respective to the stream-wise direction are 51.38° and 64.87°. In contrast to wall injection, the strut injection can enhance NG-air mixing and avoid NG penetration difficulties, although the drag of strut is large. The mass flow rate of NG depends on the injection pressure and injector numbers. Upstream or the strut within the duct flow, no NG diffuses adversely.