The invention is used in various branches of industry, in particular in agriculture for preparing seeds for sowing and for selection purposes. A method for separating a granular mixture in a flowing medium consists in 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, and the sifter is connected to a cyclone. The rotating drum is connected via an air conduit to the drive for supplying air to the generator of a cascade of flat jets. A separator of fractions which cannot pass through the sifter is in the form of a slotted confuser, a fan and a cyclone with a waste hopper, which are arranged consecutively. The claimed invention makes it possible to increase the quality of processing a granular mixture in a flowing medium, and also to use the device for separating and drying the granular mixture.