When the main and auxiliary power systems on the aircraft are unable to provide power, the Ram Air Turbine (RAT) system provides emergency power to the important equipments on the aircraft. To ensure the flight safety, the output power of the RAT generator should meet the normal working requirements of emergency loads. Therefore, the power supply capacity of the RAT generator was studied and verified through flight tests. The setting of RAT power supply capability test flight status, flight modification scheme, flight implementation process, and flight data analysis results were described, verifying the feasibility of the flight test method for RAT power supply capability.