Flame retardant is essential for cotton fabric because of the restrictions in applications caused by poor flammability. Herein, a novel flame-retardant coating assembled on the surface of cotton fabric consists of polyethyleneimine/g-C3N4 and polyethyleneimine/phytic acid as the underlying insulating coating and the upper intumescent coating, respectively. Different amounts of the composite coatings were deposited onto the fabric by the layer-by-layer self-assembled technique. The char residue of TGA over 20 wt% has indicated the significantly enhanced performance of the thermal stability of all the coated samples and the achievement of the samples with composite coatings in a self-extinguished level proving through vertical combustion test. The samples prepared with binary hybrid coatings exhibited a high limiting oxygen index value of 30.5%. From the MCC test, the peak heat release rate and total heat release of (P/G + P/PA)(4+4) showed a significant decrease of 76.9% and 77.1%, respectively. All these results demonstrated that the composite coatings endowed the cotton fabric with outstanding thermal stability and flame retardancy.