The invention belongs to the technical field of biomass recycling, and particularly relates to a method for improving the content of oil residue in water. The method comprises: (1) drying and dehydrating collected oil residue, and crushing to achieve a medium particle size (D50) of 50-300 [mu]m; (2) carrying out compression molding on the crushed oil residue obtained in the step (1) by using a sheet pressing machine or a strip pressing machine, wherein the compression pressure is 0.5-3 MPa, and the compression temperature is 30-50 DEG C; (3) crushing the oil residue obtained in the step (2) again to achieve a medium particle size (D50) of 30-50 [mu]m so as to obtain oil residue powder; and (4) mixing the oil residue powder obtained in the step (3) and water according to a certain ratio, and carrying out grinding and slurry preparing to obtain the slurry with the concentration of 40-50 wt%. The slurry prepared through the method of the invention has advantages of low viscosity and goodfluidity, wherein the process is simple, does not require additional additives, and is economical and environmentally friendly.