Contraposing the indeterminable problems such as fuzziness and diversification which influenced the slagging of coal ash, grey cluster decision was applied in evaluating the slagging properties of the power generation coal in the paper, combined with the fuzzy TOPSIS method. In the paper, the four indexes of T2, SiO2/Al2O3, B/A, G(the rate of Si) of 13 kinds of power generation coal were evaluated, the grade of slagging was divided into four grades which were non-slagging, slightly slagging, medium slagging and serious slagging. Then the grade standard value of coal slagging degree was obtained.