With the need for high speed and long range, the tiltrotor aircraft will play an increasingly important role. The tiltrotor aircraft has the advantages of vertical takeoff and landing, hovering, as well as the high speed and long range of the fixed-wing aircraft. However, its maximum speed and range is directly or indirectly limited by the aeroelastic instability (i.e., whirl flutter) of the proprotor. The whirl flutter, which can cause catastrophic aeroelastic instability, is one of the main problems in the tiltrotor aircraft design. To this end, researchers have carried out a large number of studies to avoid the occurrence of whirl flutter. In this paper, passive control methods of whirl flutter are reviewed, including the optimization of some design parameters of the proprotor and wing. The purpose is to provide a technical reserve for the future design of tiltrotor aircrafts in China.