The method of CAD-CFD analysis was used to study the design method of the theory and practice spiral case and its internal flow,the relation between the axial symmetry and uniform flow,hydraulic loss,formation of circulation and overall dimensions of spiral case were discussed,and the measures to improve its working performance were summed. The results indicate that the axial symmetry and uniform flow can be optimized by enlarging the sectional area of the nose section and the circulation can be changed by adjusting the inlet area on the basis of the traditional spiral case designed by"circulation".