Paper, as a kind of cellulosic material, is widely used in daily life. Considering the hydrophilic nature of paper, hydrophobization treatment that can improve water resistance is indispensable, especially surface sizing. In this study, an environmentally friendly sizing agent, the styrene-maleamic acid polymer containing aldehyde group (SAFA) was synthesized by styrene-maleic anhydride copolymer (SMA), 5-hydroxymethylfurfural (5-HMF, an important and commercially available bio-based chemical), and ammonia gas (NH₃). Aldehyde groups were introduced to SMA through esterification of anhydride groups and 5-HMF, and at most 50% anhydride groups could be reacted. Then, the rest of the anhydride groups were converted into amic acid groups, and the resulting water-soluble polymer (SAFA) was obtained. SAFA can be used for paper hydrophobization by dipping paper into its aqueous solution, followed by a heating treatment. The processed paper not only showed excellent hydrophobicity with a water contact angle as high as 125.6° but also exhibited strong resistance to common beverages. Furthermore, another 5-HMF-contained polymer that based on dephenolized phenol oil-maleic anhydride copolymer was also developed in the same way. These investigations provide a new technology to synthesize water-soluble polymers containing aldehyde groups and highlight the potential of SAFA as a highly promising agent for paper hydrophobization.