The disclosure provides a method for liquefying biomass, including: (a) mixing a solid organic ammonium salt containing single nitrogen with at least one organic compound which is capable of forming a hydrogen bond with the solid organic ammonium salt to form a first mixture; (b) heating the first mixture until the first mixture becomes a solution; (c) mixing a biomass and an acid catalyst with the solution to form a second mixture; and (d) heating the second mixture to make the biomass therein convert into a liquefied product.