A method for filling a thin-body flexible package provided with a spout, comprising the steps of: - providing for, inside the spout, a main duct accessible by a mouth portion and an inner chamber adjacent to the main duct and accessible by an inner opening opposite the mouth portion, the main duct leading into the inner chamber through a tubular tang wall, wherein said tang wall has a cylindrical circular inner side surface and a diameter smaller than the diameter of the main duct, the connection between the inner side surface of the tang wall and the surface of the main duct is obtained from a frustoconical connecting surface; - providing for a valve assembly comprising a normally closed valve made as a single piece and a retaining ring, said retaining ring comprising a support portion supporting the valve, and a tail portion, the retaining ring being further made as a single piece in a plastic material for supporting and applying the valve to the spout, wherein the valve comprises a flexible membrane provided with at least one through-cut forming flaps of the membrane, said valve being arranged between the main duct and the inner chamber of the spout, and wherein the valve remains pinched between the tang wall and the support portion; - providing for a filling nozzle provided with a dispensing opening arranged upstream of the valve for dispensing a product intended to fill the package; - causing the product to flow from the filling nozzle into the main duct; - deforming said flaps so as to open the valve by the action of the product in the main duct; - causing the product to flow into the inner chamber through an opening made by the flaps deformed by said product. - providing for, inside the spout, a main duct accessible by a mouth portion and an inner chamber adjacent to the main duct and accessible by an inner opening opposite the mouth portion, the main duct leading into the inner chamber through a tubular tang wall, wherein said tang wall has a cylindrical circular inner side surface and a diameter smaller than the diameter of the main duct, the connection between the inner side surface of the tang wall and the surface of the main duct is obtained from a frustoconical connecting surface; - providing for a valve assembly comprising a normally closed valve made as a single piece and a retaining ring, said retaining ring comprising a support portion supporting the valve, and a tail portion, the retaining ring being further made as a single piece in a plastic material for supporting and applying the valve to the spout, wherein the valve comprises a flexible membrane provided with at least one through-cut forming flaps of the membrane, said valve being arranged between the main duct and the inner chamber of the spout, and wherein the valve remains pinched between the tang wall and the support portion; - providing for a filling nozzle provided with a dispensing opening arranged upstream of the valve for dispensing a product intended to fill the package; - causing the product to flow from the filling nozzle into the main duct; - deforming said flaps so as to open the valve by the action of the product in the main duct; - causing the product to flow into the inner chamber through an opening made by the flaps deformed by said product.