The invention discloses a flipper with rigidity changed along with speed. The flipper comprises a flipper plate and longitudinal keels, at least one longitudinal keel positioned on the longitudinal side edge of the flipper plate is a variable-rigidity longitudinal keel with variable rigidity; the variable-rigidity longitudinal keel comprises at least one section of rigid rod and at least one rigidity speed-following adjusting device. The rigidity speed-following adjusting device is connected with the rigid rod end to end or hidden in the hollow rigid rod correspondingly; the rigidity speed-following adjusting device hidden in the hollow rigid rod comprises a hollow taper pipe in the rigid rod. The taper pipe is filled with non-Newtonian fluid or a taper rod which shares the longitudinal axis with the taper pipe and can move longitudinally along the longitudinal axis of the taper pipe, and when the rigidity speed-following adjusting device is composed of the taper pipe and the taper rod, a reset elastic element which enables the taper rod which moves longitudinally to reset is further installed between the taper pipe and the taper rod.