Fire-retardant chitosan/sodium phytate/MgO nanoparticle (CH/SP/nano-MgO) coatings were loaded on a wood substrate via electrostatic layer-by-layer self-assembly and characterized by scanning electron microscopy and energy-dispersive spectrometry. The flammability and thermal degradation of the original wood and wood samples treated with chitosan, chitosan/sodium phytate, chitosan/sodium phytate/MgO nanoparticles were studied by limiting oxygen index (LOI), exposure combustion experiments and thermogravimetric analysis (TGA), respectively. The CH/SP/nano-MgO coating served as an intumescent fire-retardant system that created a physical protection cover and exhibited the best fire retardant performance. The LOI value was 30.2% and required approximately 16–17 s to self-extinguish when exposed to air. The TGA curves also showed that char formation protected the wood from combustion.