The invention discloses a method for preparing 5-hydroxymethylfurfural by hydrothermally decomposing wood fibers, which comprises the following steps: drying and pulverizing wood fibers to 40-120 meshes, soaking with water, treating with an ultrasonic apparatus for 5-20 minutes, mixing with a catalyst and a solvent, and stirring to carry out hydrolysis reaction at 170-250 DEG C for 30-150 minutes; after the reaction finishes, cooling and filtering; extracting the water phase with an organic solvent, merging the extraction liquid and the organic phase, and removing the solvent to obtain the 5-hydroxymethylfurfural. The invention has the advantages of cheap and accessible raw material and abundant sources, can solve the problem of raw material sources of 5-hydroxymethylfurfural, relieves the environmental pollution from stacked waste wood fibers, and enhances the wood fiber hydrolysis efficiency by using ultrasonic processing.