A new method to improve the water-resistance of magnesium oxychloride composites (MOCs), with the addition of nano rice-husk ash (RHA), was studied. The related properties of samples were characterized by scanning electron microscope (SEM) and Brunauer Emmett and Teller (BET). The phase composition of MOCs was determined by X-ray diffraction (XRD). The water-resistance of MOCs was estimated by calculating softening coefficients. The experiments show that the particles size of nano RHA prepared is about 50nm, and the SiO2 content is 92.30 %; a suitable amount of nano RHA to MOCs obviously increases the water-resistance without reducing the bending strength of MOCs, and increases the softening coefficient from 0.29 to 0.78.