A laser particle size analysis method was proposed to quantitatively evaluate the fine particles coagulation. Laser particle size analyzer was used to measure the particle size distribution of coal slime at different stirring rates and different concentrations of Ca2+ or Na+. The value of d(10), d(50), and d(90) can be used to quantitatively evaluate the coagulation performance of fine particles, the greater value of d(10), d(50), and d(90) they are, and the better coagulation performance it is. It was found that the addition of Ca2+ could promote easier coagulation for fine particles than the addition of Na+, and the best coagulation performance, in which d(90) was greater than 700 mu m, occurred at the conditions of 1000rpm stirring speed and Ca2+ concentration above 5mmol/L. It is because that the electronegativity of coal and clay particles would be weakened in the presence of Ca2+, and the electrostatic repulsive force of fine particles would be reduced, resulting in the fine particles becoming easy to coagulate. Moderate stirring speed could provide kinetic energy for particle to cross the energy barrier of two colloidal particles. On the contray, coagulated particles broke up when the stirring speed reached a high level.