Ultrafine slag, which is ground with the process of vertical mill, is tested with the laser granulometric distribution apparatus quickly. The characteristics of granulometric distribution of ultrafine slag are analyzed using fractal theory. The fractal dimension of different fineness powders is tested and counted. The result shows that ultrafine slag powders have self-similar characteristics. With the laser granulometric distribution apparatus, the fractal dimension of its powders might be tested and counted with the fractal theory and its characteristics might be fast evaluated. The inherent law between powders fineness and activity is studied, and the mathematical model is built with the results. The fractal characteristics of powders might be used to meticulously evaluate fineness of ultrafine powders and be used to describe complexity of powders. The test of activation indicates that the slag fineness appraised by fractal dimension has fine linear relevant characteristic with its 7 d, 28 d activity.