The invention discloses a plant fiber-based polyol, a preparation method thereof and use thereof. The preparation method comprises the following steps: (1) separating the components of a plant fiber raw material; and (2) liquefying the plant fiber raw material. The prepared plant fiber-based polyol with a viscosity of 1,000 to 5,000mPa.s and a hydroxyl value of 260 to 500mg of potassium hydroxide/g can be used for producing rigid PU foam or semirigid foam. The method has the advantages that: raw material components are simplified by separating the components of the plant fiber raw material, so reaction is simplified, side reactions are reduced and the specificity of the liquefaction reaction is improved; hemicelluloses in the plant fiber raw material are removed, so the plant fibers are so looser as to be favorable for the permeation of a liquefying agent; the composition complexity of obtained liquefaction product is lowered for facilitating the subsequent use of the plant-based polyol; and the obtained hemicelluloses can be sold as a product along or degraded to be sold in form of monosaccharide and the utilization values of the hemicelluloses are improved.