Abstract: Starch-polyvinyl alcohol（PVA） as a slow-release fertilizer coating material has the advantages of low price and environmental protection, but its high water absorption limits its application. The use of nano-silica（SiO₂） can improve its hydrophobicity. This article first discussed the influence of PVA concentration, crosslinking agent dosage, and plasticizer dosage on the water absorption of starch-PVA composite film, then modified the starch-PVA composite film with nano-SiO₂, explored the effect of nano-SiO₂ addition on the water absorption of the composite film, found out the optimal ratio of raw materials, and finally, used the composite membrane liquid as a binder to prepare biomass carbon-based fertilizer. The test results show that when the sum of the added mass of starch and PVA in 150 mL of distilled water is controlled to be 10 g, the mass ratio of starch to PVA is 2∶3, 0.1 g of crosslinking agent borax is added, and 3 mL of plasticizer glycerin is added, the water absorption rate of the composite membrane made is the lowest at 78%. After adding nano-SiO₂, the water absorption rate of each treatment film increased first and then decreased with the increase of nano-SiO₂ addition amount. When the addition amount of nano-SiO₂ was 2.4 g, the water absorption rate of the composite film decreased 38.4%compared with that without addition, the water contact angle increased by 89.5%, and the tensile strength increased by 49.0%. The composite membrane material prepared at this time has the best performance. The nano-SiO₂-starch-PVA composite film material is applied to the preparation of biomass carbon-based fertilizer. The prepared carbon-based fertilizer particles are completely coated, and the particle size is mainly distributed between 3.8 and 4.4 mm. The cumulative nitrogen release rate at 28 d is 65%. It has a good slow-release effect and provides a new way for the development and application of carbon-based fertilizers.