Gold nanoparticles (AuNPs) with ultra-small size anchored onto support materials is highly desired towards good catalytic performance. In this study, aldehyde-functionalized chitin nanocrystals (ChNCs-PVMA) are prepared by surface-initiated electron transfer atom transfer radical polymerization (SI-ARGET ATRP) with vanillin methacrylate (VMA) as a functional monomer, which are used as reductant, stabilizer and support for the fabrication of AuNPs through an environmentally friendly process that eliminates the need for any additional reducing agents. The abundant aldehyde groups of the prepared ChNCs-PVMA are crucial to achieve ultra-small AuNPs with average size of 5.3 nm. The obtained Au@ChNCs-PVMA nanohybrid catalysts were systematically characterized by FTIR, XPS, XRD and TEM. Finally, the catalytic activity of the Au@ChNCs-PVMA catalysts are investigated for reduction of 4-NP and discoloration of azo/non-azo dyes, demonstrating excellent catalytic performance and reusability. These findings provide significant insights into the development of bio-supported nanohybrid catalyst for various environmental catalysis.