<p>Hemicellulose, a natural polysaccharide, is the second most abundant renewable component in lignocellulosic biomass. Modifying hemicellulose effectively and efficiently to achieve high-value utilization is a pressing issue in this field. In this work, hemicellulose was modified and achieved with a high degree of substitution. Phytic acid and melamine were successfully grafted onto the hemicellulose molecular chain, and the “Three sources into one” intumescent flame-retardant polymer (PHM) was synthesized. Based on the molecular design theory, the mechanistic model of the flame-retardant modification on polymers by PHM was established. The flame-retardant properties of the modified cellulose were characterized, and the flame-retardant mechanism was validated. The results showed that in FHCE-4, with PHM contents of 5.5 mass%, the heat release rate peak was 56 W g⁻¹, which was reduced by 78.6% compared to cellulose, and limit oxygen index reached 31.1%. PHM, which exhibited a three-dimensional interpenetrating network of the flame-retardant architecture, achieved efficient flame-retardant modification on cellulose. Our research provides new ideas and methods for the functional modification of hemicellulose, offering more possibilities for its high-value utilization.</p>