The invention is used in various branches of industry, in particular in agriculture for preparing seeds for sowing and for selection purposes. The method of separating a granular mixture in a flowing medium is based on a gravitational supply of particles, with an aerodynamic, monotonously increasing action thereon with a cascade of flat jets, said cascade being stabilized in terms of pressure and discharge. Large impurities are removed from the granular mixture beforehand, and stable parameters are maintained in the separation chamber. The device comprises a vibrating screen, an adjustable hopper with an agitator and a vibrating chute, a generator of a cascade of flat jets, which is connected to a drive for supplying air under pressure, a fraction collector and a separation chamber. The output of the separation chamber is covered with a filtering element in the form of a rotating drum with a calibrating sifter on the surface, said sifter being connected to a cyclone.