The invention relates to a method for transforming biomass sugar source into 5-hydroxymethyl-furfural, in particular to a method for preparing the 5-hydroxymethyl-furfural. The method comprises the following steps: using ionic liquid as a solvent, hexose or hexose source biomass as raw material substrates, and 0.5 to 50 percent (relative to the mass of the biomass sugar source) of acidic ionic liquid, inorganic acid or organic acid as a catalyst, and performing the reaction of materials for 1.5 minutes to 23 hours at normal pressure and at a temperature of between 80 DEG C and 100 DEG C to efficiently generate HMF. The method has the advantages of high HMF selectivity, less acid consumption, moderate operating conditions, fast reaction, reusable ionic liquid, simple process, environment protection and the like, and opens up a new approach for preparing commodity chemicals and replacing fuels starting from renewable biological resources.