A preparation method of a soluble dietary fiber comprising: subjecting lignocelluloses to continuous countercurrent leaching, separating and obtaining a solid material, wherein the specific operation of the continuous countercurrent leaching comprises that: the lignocellulose raw material is pulverized into particles having a length or particle size of 0.1 cm to 5 cm, placed in a continuous countercurrent extractor, a solvent medium(s) is added in such an amount that the ratio of the raw material to the solvent medium is 1 g: mL, wherein the controlled temperature is 30 °C to 90 °C and the extraction time is 20 to 60 minutes, then the solid material and the leach liquor thus obtained, which is rich in monosaccharides, are separated by continuous solid-liquid separations; subjecting the solid material obtained in step to continuous steaming processing in a transverse pipe, obtaining a steamed material, wherein the continuous steaming processing in a transverse pipe comprises that: the solid material is pushed into a transverse pipe through a screw conveyor, a material plug is formed, steam is introduced, they are thoroughly mixed and steamed under the tumbling action of the screw conveying and the steam to efficiently dissolve the hemicellulose form the raw material; the steaming pressure is 0.2 to 1.0 MPa, and the steaming time is 10 to 90 minutes; subjecting the steamed material obtained in step to enzymolysis by xylanase, wherein the steamed material is subjected to pressure relief and is sprayed into an enzymolysis tank, water is added to the enzymolysis tank, the mass ratio of material to water is adjusted to 1:, and the pH is adjusted to 3.5-6.5, xylanase is added for enzymolysis in an amount of 1 × 10 4 U to 8 × 10 4 U of xylanase per kilogram of dry matter, enzymolysis reaction is carried on to obtain a soluble dietary fiber crude sugar liquor; wherein the xylanase is a xylanase prepared by fermentation with a microbial strain or an endo-xylanase commercially available, the temperature of the enzymolysis reaction is 40 °C to 80 °C, and the reaction time is 4 h to 20 h; subjecting the crude sugar liquor obtained in step to decolorization, desalination and filtration, obtaining a soluble dietary fiber sugar liquor; and concentrating or drying, obtaining a soluble dietary fiber I; in the soluble dietary fiber I, the mass percent of content of xylooligosaccharides in total sugar content is 80% or more; and in the xylooligosaccharide, the total content of xylobiose and xylotriose is 55% or more. subjecting lignocelluloses to continuous countercurrent leaching, separating and obtaining a solid material, wherein the specific operation of the continuous countercurrent leaching comprises that: the lignocellulose raw material is pulverized into particles having a length or particle size of 0.1 cm to 5 cm, placed in a continuous countercurrent extractor, a solvent medium(s) is added in such an amount that the ratio of the raw material to the solvent medium is 1 g: mL, wherein the controlled temperature is 30 °C to 90 °C and the extraction time is 20 to 60 minutes, then the solid material and the leach liquor thus obtained, which is rich in monosaccharides, are separated by continuous solid-liquid separations; subjecting the solid material obtained in step to continuous steaming processing in a transverse pipe, obtaining a steamed material, wherein the continuous steaming processing in a transverse pipe comprises that: the solid material is pushed into a transverse pipe through a screw conveyor, a material plug is formed, steam is introduced, they are thoroughly mixed and steamed under the tumbling action of the screw conveying and the steam to efficiently dissolve the hemicellulose form the raw material; the steaming pressure is 0.2 to 1.0 MPa, and the steaming time is 10 to 90 minutes; subjecting the steamed material obtained in step to enzymolysis by xylanase, wherein the steamed material is subjected to pressure relief and is sprayed into an enzymolysis tank, water is added to the enzymolysis tank, the mass ratio of material to water is adjusted to 1:, and the pH is adjusted to 3.5-6.5, xylanase is added for enzymolysis in an amount of 1 × 10 4 U to 8 × 10 4 U of xylanase per kilogram of dry matter, enzymolysis reaction is carried on to obtain a soluble dietary fiber crude sugar liquor; wherein the xylanase is a xylanase prepared by fermentation with a microbial strain or an endo-xylanase commercially available, the temperature of the enzymolysis reaction is 40 °C to 80 °C, and the reaction time is 4 h to 20 h; subjecting the crude sugar liquor obtained in step to decolorization, desalination and filtration, obtaining a soluble dietary fiber sugar liquor; and concentrating or drying, obtaining a soluble dietary fiber I; in the soluble dietary fiber I, the mass percent of content of xylooligosaccharides in total sugar content is 80% or more; and in the xylooligosaccharide, the total content of xylobiose and xylotriose is 55% or more.