Abstract: A whirl flutter of a propeller rotor system has been investigated experimentally in the wind tunnel.The design of whirl flutter experimental equipment and a dynamic stiffness coefficient measurement method for the supporting system are also studied.The experiments show that if the support stiffness of the propeller rotor system is reduced as a result of failure of some elements in the engine frame or mount,the threshold flight velocity of onset whirl flutter will be decreased.The results of its wind tunnel experiments exhibit the same trends as in its theoretical calculation.