Abstract: Currently the study on the relationship between the control of flocculation kinetics,the change of flocculation morphology and the quality of ultra-clean coal by physical method is limited. In this paper,the fractal dimension theory was used to analyze the nonlinear changes of the morphology and diameter size of the particles in the shear force field and used to study the separation effect of ultra-clean coal. According to the mathematical analysis on the density ex- pressions of flocculation,it was verified that the density of flocculation was the function of fractal dimension and the in- put of mechanical energy. Through the study of the two-dimensional fractal dimension,the whole and the average of the particle population are described and characterized. By analyzing the multifractal spectrum of the flocculation,the dy- namic process of the fractal changes of the clusters is revealed. In addition,the flocculation kinetics control factors were proposed which were expected to contribute to the scientific basis for the process operation of ultra-clean coal preparation and reasonable guidance for practical engineering application. The study shows that when the stirring speed is 2 000 r / min,the two-dimensional fractal dimension of flocculation has a maximum of 1. 762 that means at this point the flocculation has a dense structure and appropriate size,the characteristic parameters of multifractal spectrum like f(αmax ) has a maximum value of 0. 796,at this point the flocculation has largest size and maximum amount. Therefore under this condition the ultra-clean coal has the highest yield of 84. 314% and the lowest ash content of 0. 831% .