Composite materials using natural fibers (NFs) are of increasing interest due to their carbon neutrality, high strength, lightness, and biodegradability. However, when NFs are applied as a reinforcing agent for the existing petroleum-based plastic matrix, the lack of suitable polymer processing technology inevitably leads to a decrease in compatibility. In this study, a high-strength NF preform was prepared as a core layer for sandwich-structured fiber-reinforced plastic (FRP) manufacturing. To address this, cellulose nanocrystals were used as a coating and interfibrillar adhesive to increase the formability of the nonwoven preform and increase its mechanical properties. Additional drying and wetting processes were proposed to prevent the loss phenomenon in the hot-pressing process due to the low viscosity of cellulose nanocrystals. Through this, the content of the remaining cellulose nanocrystals of the NF preform was increased, and the mechanical reinforcement effect was realized accordingly. Eventually, when the NF preform was used as a sandwich-structured biodegradable FRP composite, the excellent mechanical properties of the composite were also expressed effectively.