Abstract: Gravity separation is the main method of coal preparation, therefore, it is important to evaluate and predict the effect of coal preparation. However the key problem of gravity effect prediction is how to determine the distribution rates of the density grades. It is complicated to predict the rate of product quality and quantity through the existing calculation methods, which should calculate t through the approximate formula and then check the distribution rates of the density grades through distribution index table. It is found that a mathematical model of "S" shaped curve based on the research of the distribution curve was established to predict the product of the gravity separation of coal, which can evaluate the effect of gravity separation conveniently and accurately.