Flame-retardant flax fabrics were developed using the grafting of phytic acid (PA) through a two-step process. First, flax fabrics were functionalized with glycidyl groups employing the polymerization of glycidyl methacrylate (GMA) via a simultaneous e-beam irradiation procedure. Subsequently, the modified flax fabrics were treated with PA using two different techniques: the first method involved a direct grafting approach, where the phosphonic functions of PA reacted with the glycidyl groups. The second technique employed a coating process, which was based on a combination of PA and ethylene glycol diglycidyl (EGDG) as a curing agent, producing a phosphonated layer on the flax fibers. The treatment processes were applied to pristine and alkali pretreated flax fabrics, allowing an investigation into the impact of delignification and PA grafting on burning behavior. SEM-EDS, 13C and 31P solid NMR, and FTIR were used to characterize the treatment process steps. Thermogravimetric analysis (TGA), pyrolysis combustion flow calorimetry (PCFC), and cone calorimetry tests were performed to evaluate the pyrolysis and combustion behaviors of prepared samples. The treated flax fabrics exhibited a good flame-retardant property only at 1.3 wt % of P content, while for a lower content, it was not effective enough. Moreover, the delignification of the flax fabric contributed significantly to enhance its fire behavior.