The stone floating in the rapids and airplane rising in the air should follow the same principle. Through analyzing and calculating the problem that high-speed water flow makes the stone float, puts forward the concept of dynamic buoyancy, proves that dynamic buoyancy is the cause of stone floating, and applies this result to the problem of aircraft lift, the calculation formula of aircraft lift is derived, namely FX=ρv2Sx/2+ρgV, and solves the defects of current lift theory. Dynamic buoyancy is the cause of the lift produced by the aircraft.