<p>Compared with other transparent films, cellulose films have more advantages in terms of cost and environmental friendliness. Nowadays, transparent cellulose films are used in food packaging, optoelectronic devices and other applications. However, the flammability of cellulose limits its further application. In this study, based on the NaOH/urea/H₂O low temperature dissolving system of cellulose and the water solubility characteristics of flame-retardant low-molecular-weight ammonium polyphosphate (LAPP), a series of flame retardant cellulose films were prepared by diffusing the LAPP solution into the regenerated cellulose (RC) hydrogel films and drying. The content of LAPP in RC/LAPP composite film can be regulated according to the concentration of LAPP in the diffusion solution. When the LAPP concentration in the diffusion solution was 0.5&#xa0;wt.%, the LAPP content in the RC/LAPP composite film reached 4.25&#xa0;wt%. Meanwhile, the limited oxygen index of the RC/LAPP composite film was 27% and the vertical combustion grade reached V-0, while the transparency and the mechanical properties of the film did not deteriorate. LAPP flame retardant cellulose was achieved mainly by condensed phase flame retardation. It is expected that this method can be extended to the preparation process of other functional transparent films.</p>