Layer by layer (LBL) assembly technique was used to deposit multilayer coating containing phosphorus-nitrogen onto the surface of fibers to improve the flame retardancy of polyester-cotton (PET-COT) blended fabric. Ellipsometer results confirmed that polyallylamine hydrochloride (PAH), melamine (MEL), and ammonium polyphosphate (APP) grew linearly on silicon wafer during the LbL process. The LOI value of coated PET-COT fabric was increased from 20.8% of pristine fabric to 28.4% by the presence of about 9 wt% coating. Besides, this intumescent nanocoated PET-COT fabric was self-distinguished during the vertical burning test. Thermogravimetric analysis under both nitrogen and air atmosphere revealed that the initial degradation temperature of the coated sample was decreased and the char residue amount was significantly improved during combustion. The flame resistance performance evaluation by pyrolysis combustion flow calorimeter indicated that this coating effectively reduced the peak heat release rate of PET-COT matrix. The scanning electron micrographs of char residue demonstrated that the char formation in the condensed phase and free radical caption in the gas phase was responsible for the improved flame retardancy. It is suggested this unique facile coating technology with low cycles and high efficiency has great potential to produce commercially available flame retardant polymeric-cotton blend fabrics.