Natural cellulose fabrics, as one of the combustibles prone to causing indoor fires, are modified with flame retardants encouraged by the fire protection industry. Herein, a bio-based colloid named ASDPC was prepared by 3-aminopropyltriethoxysilane, diethylenetriaminepentakis(methylphosphonic acid) and β-cyclodextrin, and used to enhance the flame retardancy of flax fabrics via the pad-dry-cure method. The treated flax fabrics named ASDPC-FF showed good flame retardancy with a damaged length of only 6.4 cm in the vertical burning test and a limiting oxygen index value of up to 33.8 %. Meanwhile, ASDPC-FF exhibited thermal stability and combustion behavior distinct from untreated flax fabrics. Thermogravimetric analysis presented that the char residue produced by ASDPC-FF at 800 °C was as high as 36.2 %. Cone calorimetry test indicated that the peak heat release rate and total heat release of ASDPC-FF decreased by approximately 89.0 % and 35.7 %, respectively. Furthermore, compared to untreated flax fabrics, condensed-phase and gas-phase studies found that ASDPC-FF turned into expanded char and produced fewer gaseous volatiles containing C-H, CO and C-O-C at high temperatures. More importantly, ASDPC-FF remained self-extinguishing after 25 laundering cycles with flame retardancy surpassing untreated flax fabrics. In summary, this work provides a novel route for expanding the manufacture of flame-retardant flax fabrics.