•Eco-friendly PAN composite was prepared by using biomass materials.•PAN composite had excellent flame retardancy and antibacterial properties.•The modification is environmentally friendly, simple process and easy to prepare. To improve the fire safety of polyacrylonitrile (PAN) and expand its application area, flame retardant and antibacterial PAN composite was prepared by a simple and environmentally friendly method. First, AgNO3 and NaBH4 solutions were added to the microcrystalline cellulose (MCC) aqueous suspension to prepare MCC loaded Ag nanoparticles (Ag NPs) (MCC/Ag) by in situ reduction. Then, MCC/Ag was mixed in PAN solution to prepare MCC/Ag/PAN composite, which was then phosphorylated by phytic acid (PA) to obtain flame retardant MCC/Ag/PAN composite (FR-MCC/Ag/PAN). FR-MCC/Ag/PAN could achieve self-extinguishing with limiting oxygen index (LOI) value of 36.8 %. The thermogravimetric results showed that phosphorus-containing acids accelerated the char-forming ability of FR-MCC/Ag/PAN, and the final char residue was up to 29.76 % in air atmosphere. Thermogravimetric coupled with Fourier transform infrared spectroscopy (TG-FTIR) and scanning electron microscope (SEM) showed that FR-MCC/Ag/PAN formed an expanded and dense char layer after combustion, suppressing the release of low molecular toxic gases. Moreover, FR-MCC/Ag/PAN showed favorable inhibition of Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). This work proposed a novel method for developing flame retardant and antibacterial PAN material, which broadened the application field of polyacrylonitrile.