Aiming at the problem of lacking of effective cutting particle diameter prediction method in the cut cyclone design process, two key indexes affecting the change of cutting particle diameter are proposed in this paper. The flow field characteristics of the cut cyclone under 30 L/ min volume flowrate are studied with CFD method, through analyzing the performance changes of the cutting efficiency, median particle diameter, pressure drop and etc. caused by the structural dimensions of cylinder and cone sections, considering the two key indexes of the centrifugal effect ψ and structural height to cyclone length ratio Y/ l, the changing law of the structural dimensions affecting the cutting particle diameter is obtained, and the results agree with the experiment trend in the reference article. The study shows that when Y/ l≤70% , increasing Y/ l can improve the cutting performance; increasing the ratio of conical section to total height can improve the cutting performance while reducing the structural resistance; increasing the inner surface area of the cut cyclone can reduce the structural resistance. Starting from studying the changes of the two key indexes, this paper provides a reference for the cut cyclone structure design and the precision control of cutting particle diameter.