In order to research the variation of water droplet size in the aero-engine rain ingestion test, the numerical simulation method of DPM model was used to investigate the variation of water droplet size in the inlet. The engine was operated at different rating during the rain ingestion simulation test. of the engine under different engine rating during the rain ingestion simulation test. The calculation results show the change of droplet size and distribution after water droplets enter the inlet. The results show that under different rating states, the VMD (Volume Mean Diameter) and SMD (Sauter Mean Diameter) values of water droplets with different initial sizes decrease sharply and remain constant when they enter the inlet. In the high rating state, the water droplets are injected at a 50°conical angle in front of the inlet lip. The water droplets converge to the central axis of the inlet along with the air flow, and there is interference between the water droplets emitted from different nozzles. In the idle state, the VMD and SMD values are larger than those in the max status, a part of the water droplets from the nozzle near the edge of the inlet will hit the lip and splash out of the inlet.