Herein, the acid-leaching treatment and surface silanization of catalytic gasification ash were used to modify catalytic gasification ash (Raw-Ash), and the modified Raw-Ash was used to prepare ASH/polyurethane (PU) composites. The effect of acid-leaching treatment and surface silanization on the properties of Raw-Ash and resulting ASH/PU composites was investigated systematically. The results show that the acid-leaching treatment greatly increased the specific surface area of the acid leached catalytic gasification residue (H-Ash), so there can be more positions between the PU matrix and the filler and where a bond can be formed. In the surface silanization process, a layer of polysiloxane coating was grafted on the surface of H-Ash, which dramatically improved the interfacial adhesion between the fillers and PU matrix, and enhanced the mechanical properties of the composites, namely, 35.0% of tensile strength, 6.9% of elongation at break, and 69.0% of tear strength.