Abstract: The particle size distribution has an important influence on the preparation of coal water slurry（CWS）. Based on the existing evaluation model of packing density, the thickness of the water layer around the particle is proposed by theoretically calculating with the particle expansion rate. By using this method, four kinds of samples with equal packing density but different thickness of water layer were obtained by controlling the particle size distribution of a long flame coal. The coal water slurry were prepared, the concentration and stability were tested and analyzed. The results show that while the packing density is 84.61±0.05%, the concentration increases linearly with the increase of water layer thickness, but its value is distributed within 59.95±0.3%. The packing density is still effective of the preparation of CWS. The backscattering spectrum test results show that the content of free water contained in the three-dimensional structure of coal, water and dispersant formed in system increases due to the increase of water layer thickness, and resulting that the water cleavage rate increases. The migration range of the particles in the system increases, causing the aggregation phenomenon of the whole system to be aggravated, and the stability is lowered.