In aero-engine test runs, the humidity of inlet air will affect the accurate evaluation of engine performance. The design and research of air atomization humidification system was carried out for an aero-engine test bench. The movement process and mass transfer process of droplets was studied by using the calculation method of gas-liquid two-phase flow based on Euler method, and the influence of droplet parameters on air humidity was analyzed. The results show that the droplet diameter directly affects the inlet air humidity. Under the condition of saturated steam humidification flow, the air temperature has a parabolic relationship with the maximum droplet diameter. In low temperature environment, the inlet air humidity can be increased by spraying excessive water flow. According to the analysis results, the atomization nozzle was selected and the layout of nozzles was designed for an aero-engine test bench. Different tempera inlet air can be humidified by opening different numbers of nozzles.