Surface of nanometer silicon dioxide and magnesium hydroxide was modified by Solsperse17000 dispersing agent. Nano - concentrates were gotten by mixing modified nanoparticles with resin through ball milling and supersonic wave, then flame retardant nano - coatings with different content of nano - concentrates were prepared. Dispersion morphology and stability principle of nanoparticles were analyzed by the use of transmittance electron microscopy （TEM） and Fourier transform infrared spectroscopy（FT- IR）. Morphology, molecule structure, water transport behavior and thermal degradation of the flame retardant coatings were studied by use of SEM, FT - IR, curve of weight loss, DTA. It is indicated that 4 % nano - concentrates can enhance water resistance and do not harm thermal degradation and flame retardant property.