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. <EOS>