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Due to the high flammability of lignocellulose filler and plastic matrix, wood-plastic composites are easily ignited with large amounts of heat and smoke, which has the potential risk of causing fire. In this study, sisal fiber was modified with ammonium polyphosphate, polyethyleneimine and nano-silica based on the attraction between oppositely charged polyelectrolytes. FTIR, XPS and SEM indicated that the flame retardant was successfully introduced into the lumen and surface of fiber, and its content increased with the increase of modification times. Then, the modified fibers with different contents of flame retardants were dispersed into polypropylene and a three-dimensional flame retardant network was constructed. The results of TG, LOI and combustion testing showed that the fire retardance of the fiber and composites improved with the increase of fiber modification times. After 4 cycles of modification, the LOI of fiber and composites increased from 20.7% and 18.9% to 37.8% and 22.8%, respectively. The morphology of the fiber and composites after combustion illustrated that the modified fiber can act as network skeleton and promote the construction of char layer. Consequently, the combustion and heat release of fiber were inhibited by flame retardant modification, and the flame retardant network also hindered the combustion of the polypropylene matrix, which effectively improved the fire retardance of composites. • Sisal fiber with multiple lumens was used as the carrier of flame retardants. • The cross-dispersion between fibers was utilized to build a flame retardant network. • This network can act as a skeleton and promote the carbonization of surrounding PP. • Carbonized PP wrapped around the network skeleton to form a dense carbon layer. [ABSTRACT FROM AUTHOR]