For researching phenomena of the cavitation and the air ingestion in the nozzle under different ambient pressures, a visualization experiment with a transparent injector nozzle was carried out, which used the stroboscope, long-distance microscope, CCD camera, high-pressure spray chamber etc. The VOF method and the overset grid technology were used to simulate transient state flow in the spray orifice and the sac. The simulation results showed that the strong cavitation phenomenon would happen in the orifice and the sac of nozzle after the end of injection. The void volume of the cavitation bubble collapsing was filled with ingested air. And the cavitation volume was roughly equal to the ingested air volume based on the calculated data which can prove the conclusion. This showed that the cavitation collapse was the main cause of air ingestion. The cavitation collapse in the sac was a necessary condition for the ingestion air further flowing into the sac. And the smaller the cavitation area in the sac was, the less hot combustion gas was ingested into the sac. The cavitation number was proposed which represented the degree of cavitation in the nozzle after the end of injection. Further studies found that an increase in ambient pressure can cause the cavitation number to increase which led to the cavitation volume and the ingested air volume tend to decrease. And the cavitation number explained the influence of ambient pressure on air ingestion, and this was consistent with the experiment and simulation results.