Abstract: The rice straw/magnesium oxychloride cement (MOC) inorganic light composites were prepared with hydrogen peroxide solution as the foaming agent, MnO₂ as the excitation agent, calcium stearate as the stabilizing foam agent, polyacrylamide as the thickening agent. The effect of straw size and adding quantity on the dispersion of straw in the slurry and its effect on the strength, toughness, crack resistance and water absorption of straw/MOC composites were discussed. The results show that the straw size and the amount of addition have a great influence on the straw dispersion and strength of straw/MOC composites. When the straw size is smaller than 250 μm and the quantity is 0.9% mass ratio of straw to MOC, the straw dispersion is the best, the compressive strength (11.26 MPa), bending strength (3.97 MPa) and anti-crack of straw/MOC composites are the largest. SEM analysis show that there are weak layer between straw and the matrix because the poor cementing quality. When the straw size is too large or straw quantity is too much, per unit volume of the straw/MOC composites will be introduced more weak interfacial bonding part. As a result, the overall performance of the sample will be affected.