Fire accidents can trigger severe personal injury and large-scale property damage. Therefore, the use of flame retardant is essential. A novel green ternary nanocomposite intumescent flame retardant (IFR) (CNC@MEL-PA) was fabricated by successively modifying the gas source melamine (MEL) and acid source phytic acid (PA) on the carbon source cellulose nanocrystals (CNCs). CNC@MEL-PA exhibited better thermal stability, carbon layer formation ability, and smoke suppression efficiency due to its characteristic two-dimensional (2D) petal structure and intumescent carbon layer. Notably, compared to CNCs, the expanded carbon layer volume of CNC@MEL-PA was enhanced from about 62.2 to 169.6 mm3. Therefore, this study provides a facile synthetic route of high-performance CNC@MEL-PA with good smoke suppression efficiency for a green flame-retardant material for construction and transportation products.