Abstract: Nanocellulose, recognized for its renewable nature, excellent mechanical strength, high specific surface area, and tunable surface chemistry, has emerged as a key building block for the next generation of intelligent and green agricultural composites. This review systematically outlines the structural characteristics of nanocellulose and its design strategies for composite formation, elucidating the mechanisms and approaches by which it serves as a building block to achieve functional enhancement and performance regulation of composite materials. Recent advances in nanocellulose-based composites are critically assessed, with particular attention to applications in controlled-release fertilizers, targeted pesticide delivery systems, biodegradable agricultural mulch films, and soil remediation. This review identifies the current major challenges in scalable production, long-term environmental safety assessment, and establishment of standardization systems. Finally, future prospects for high-performance, environmental multi-responsive composite materials designed to meet the demands of smart agriculture are discussed. This review aims to provide theoretical insights and material design guidelines to support the transition toward greener agricultural practices.