The load capacity of airplane爷s ram air turbine (RAT) depends not only on airplane airspeed, but also on the flight attitude. It is difficult to calculate by using theoretical equations. A flight test method is designed to determine the RAT's real-time load capacity by just analyzing some flight test data obtained. This method plans two test subjects: RAT load capacity test at level flight and RAT load capacity test at sideslip. The former test can obtain the variation of RAT airspeed with airplane airspeed for different weights, and the latter can bring us the relationship for different sideslip angles. The results of the two tests show that the RAT airspeed at any weight and flight attitude can be determined and further know its load capacity.