Objective To study the preparation of hydrophobic agent by surface modification. Methods Using silane coupling agent as the surface finishing agent, hydrophobic fumed silica power as the functional filler, a hydrophobic agent was prepared for wrapping paper and cardboard. The contents of each component and the curing processing of the hydrophobic agent were investigated. Results The optimal formula ( mass fraction) determined was, silane coupling (2. 0% ) , fumed silica power ( 0. 8% ) , and allowance for anhydrous ethanol. After curing the prepared sample at 120℃ , its contact angle was measured as 138. 3^o , the coating showed strong adhesion and had no influence on the appearance of cardboard. Conclusion The hydrophobic agent consisting of silane coupling agent and fumed silica power can effectively improve the hydrophobicity of the paper surface, and the curing processing has a significant role in improving the transparency and adhesion of the coating.