Abstract: Coal water slurry( CWS),a coal-based liquid fuel with similar physical properties to oil,is an important way for efficient and clean utilization of coal,which can significantly reduce the emission of NOxand SO_2. At present,China's CWS demand has exceeded 250 million tons/year. Particle size gradation technology is an important factor affecting the concentration of coal water slurry,as different sizes of coal particles can be filled with each other to improve the packing efficiency of particles and the concentration of CWS. This paper analyzes the influence of particle gradation on CWS,and points out that in the CWS making process,the particle gradation only involves physical crushing and grinding,and the energy consumption is relatively small,which has significant effect on improving the concentration and has wide applicability and economy. The theory and model of particle gradation developed in recent years,especially the interlayer packing theory and fractal gradation theory,are discussed,and the specific calculation method,formula,hypothesis and applicability are given. The development and application of the interlayer packing theory in recent years and the evaluation of the packing efficiency are given by examples. The feasibility and new progress of fractal gradation theory and its application in bituminous coal and lignite CWS are analyzed. The technological process and industrial practice cases of three-peak gradation are given. Scholars have done a lot of work on the basic theory and application of particle gradation to improve CWS concentration. However,in following specific theoretical details need to be further studied: the simplified calculation of interlayer packing for arbitrary particle size distribution,the fitting accuracy of fractal gradation and the theory of three-peak gradation. In the application of gradation theory,the combination of basic theory and industrial practice needs to be further improved. It is suggested to introduce the influence of internal water and establish a prediction model for different types of coal in the future research.