A method for manufacturing a multi-layered paperboard, which comprises at least two fibrous layers and which layers are formed by multiple separate forming units, wherein at least part of water is drained from at least one layer, the layers are joined together, and the joined layers are subjected to further draining, wet-pressing and drying for obtaining the multi-layered paperboard product, and in which method at least one fibrous layer of the multi-layered paperboard is treated by applying an aqueous solution of a first strength component in dissolved form on a surface of the layer, which surface is arranged to be in contact with other layer of the multi-layered paperboard to be produced, prior to joining the fibrous layers together, and an aqueous solution of a cationic second strength component in dissolved form is added to the fibre stock from which at least one of the fibrous layers joined together is formed, characterised in that - the first strength component comprises anionic strength polymer and/or amphoteric strength polymer composition having a net charge of -0.1 - -3.0 meq/g (dry) at pH 7, and the composition comprising said first strength component has a viscosity of 1.4 - 100 mPas measured by Brookfield LV DV1 viscometer with small sample adapter using maximum rpm allowed by the equipment, instantly after mixing, at prevailing temperature and solids content at the application time, wherein - the anionic strength polymer comprises anionic vinyl polymer, carboxymethyl cellulose or any combination thereof, and - the amphoteric strength polymer comprises amphoteric vinyl polymer, or a combination of anionic strength polymer(s) and cationic strength polymer(s), wherein the anionic strength polymer comprising anionic vinyl polymer, carboxymethyl cellulose or any combination thereof, and the cationic strength polymer comprising cationic vinyl polymer, cationic starch, polyamine or any combination thereof, - the cationic second strength component comprises cationic starch having cationic degree of substitution 0.015 - 0.06, and/or synthetic cationic strength polymer having weight-average molecular weight of 400 000 - 3 000 000 Da measured by GPC SEC polyethyleneoxide PEO calibration, and charge density of 0.5 - 4 meq/g (dry) at pH 7, and wherein the cationic second strength component is added in an amount of 2-25 kg/ton of fibre stock as dry in case of cationic starch or 0.7-5 kg/ton of fibre stock as dry in case of synthetic cationic strength polymer, and in that an aqueous solution of a first strength component is applied on a surface of the layer, when the dryness of the fibrous layer is 2 - 18 %, expressed as an amount of water removed from the fibrous layer. - the first strength component comprises anionic strength polymer and/or amphoteric strength polymer composition having a net charge of -0.1 - -3.0 meq/g (dry) at pH 7, and the composition comprising said first strength component has a viscosity of 1.4 - 100 mPas measured by Brookfield LV DV1 viscometer with small sample adapter using maximum rpm allowed by the equipment, instantly after mixing, at prevailing temperature and solids content at the application time, wherein - the anionic strength polymer comprises anionic vinyl polymer, carboxymethyl cellulose or any combination thereof, and - the amphoteric strength polymer comprises amphoteric vinyl polymer, or a combination of anionic strength polymer(s) and cationic strength polymer(s), wherein the anionic strength polymer comprising anionic vinyl polymer, carboxymethyl cellulose or any combination thereof, and the cationic strength polymer comprising cationic vinyl polymer, cationic starch, polyamine or any combination thereof, - the anionic strength polymer comprises anionic vinyl polymer, carboxymethyl cellulose or any combination thereof, and - the amphoteric strength polymer comprises amphoteric vinyl polymer, or a combination of anionic strength polymer(s) and cationic strength polymer(s), wherein the anionic strength polymer comprising anionic vinyl polymer, carboxymethyl cellulose or any combination thereof, and the cationic strength polymer comprising cationic vinyl polymer, cationic starch, polyamine or any combination thereof, - the cationic second strength component comprises cationic starch having cationic degree of substitution 0.015 - 0.06, and/or synthetic cationic strength polymer having weight-average molecular weight of 400 000 - 3 000 000 Da measured by GPC SEC polyethyleneoxide PEO calibration, and charge density of 0.5 - 4 meq/g (dry) at pH 7, and wherein the cationic second strength component is added in an amount of 2-25 kg/ton of fibre stock as dry in case of cationic starch or 0.7-5 kg/ton of fibre stock as dry in case of synthetic cationic strength polymer,