Abstract: An experimental study has been carried out on the effect of high ambient air pressure on the spray characteristics of a plain jet injector in coaxial high velocity airflow. The air pressures tested range from 1 to 16 atm, the airflow velocity from 60 to 120rn/s and the pressure drops of the injector tested from 0.2 to 2 MPa, Working fluid is water. The key feature of the experiment is to use a convergent-disvergent nozzle to maintain a high air pressure in the test chamber and at the same time to maintain a high airflow velocity in the atomi-zation zone. It totally eliminates the air recirculation and problem of slow droplet settling in a commonly used pressurized vessel for high pressure atomization. The results show that SMD decreases monotonuously with the increase of ambient air pressure, and at different air velocities the effect of the air pressure is different.The drop size distribution parameter N in Rasin-Rammler distribution decreasesslightly with the increase of ambient air pressure.