Based on the pneumatic mechanics and the gas-solid two-phase fluid theory, a model of gas-solid two-phase flow speed field was set up for horizontal turbines during the classifying process, and the formula of classified particle cut size in irregular speed fields was deduced. In order to evaluate the model, a number of experiments were carried out in a fluidized-bed jet-mill using samples such as calcium carbonate, carbon silicon, alumina, etc. It was found that the formula of classified particle cut size agreed with the experimental results very well. Furthermore, the relationship between the structural parameters of turbine and the classifying results was discussed, and the turbine structure was optimized according to the formula of classified particle cut size.