Abstract: Establishing double-variant mathematical model for the partition surface is an effective method for accurately describing the partition law of the particles with different density and different size as well as for quantitatively analy- zing the influence of particle density and size on the cyclone separation efficiency. To improve the adaptability of the mathematical models to different types of cyclones,a combination method of density and size variables was proposed based on the terminal settling velocity formula over a wide range of Reynolds number,and a six-parameter mathemati- cal model for the density-size partition surface was established taking normal distribution as a fundamental function,in which the meaning of the model parameters were clarified. From the model,it could be deduced that the separation density and the ecart probable are inversely proportional to the particle size. The model data has high accuracy to ap- proach the separation results of classifying cyclone,water-only cyclone and dense medium cyclone,which tentatively reveals the binary influence mechanism of density and size in cyclone separation process.