This paper presents the simulation study of internal flow of open-end swirl injectors under steady and oscillating ambient pressures. A two-dimensional swirl axisymmetric model based on the volume of fluid method was developed to study the effect of ambient pressure on the internal flow of open-end swirl injectors. The response of injector flow to the ambient pressure oscillation was investigated by superimposing periodical oscillation of ambient pressure at the spout outlet. The results show that the variation of ambient pressure affects the liquid phase volumetric fraction within the gas–liquid shear layer. The spray angle near the wall remains constant independent of the ambient pressure. The velocity distribution on different axial sections rarely varies with ambient pressure. When the ambient pressure oscillated, the ambient pressure oscillation would cause the flow rate oscillation at the spout. The phase delay between the flow rate oscillation at spout and the ambient pressure oscillation is proportional to the oscillation frequency.