The invention relates to a method for removing lignin from lignocellulosic biomass. The method comprises feeding of lignocellulosic biomass and a first aqueous solution into a reactor vessel, the lignocellulosic biomass and the first aqueous solution forming a reaction mixture; reducing the pressure in the reactor vessel below 0.8 bar absolute pressure, preferably 0.5 bar absolute pressure, more preferably below 0.2 bar absolute pressure; keeping the reaction mixture in a predetermined extraction temperature, and adding at least one extraction chemical, such as base or acid, to the reaction vessel and extracting lignin from the biomass to the liquid phase of the reaction mixture.