A basic equation system for meridional throughflow fields in multistage axial flow compressors has been deduced,containing many unknown correlation terms,which describe different kinds of spanwise mixing mechanism in aunified form.The equation system shows that spanwise mixing of meridional flows in compressors is attributedto three kinds of mechanism:molecular motion,turbulent diffusion,and circumferential non-uniformities,thelast of which includes secondary flow effects and others.Therefore the equation system unifies the two modelsfor spanwise mixing analyses by Adkins & Smith (1981) and Gallimore & Cumpsty (1986).With three kindsof apparent mixing coefficients defined and introduced into the basic equation system,a novel,much simplerequation system,without additional unknown correlation terms included,has been obtained.This novel equa-tion system makes throughflow computations including mixing far easier for multistage compressors.It hasbeen rigorously shown that these apparent mixing coefficients contain full information of all the three kinds ofmixing mechanism,so that the simpler equation system can also be taken as a unified model for meridional flowswith all the kinds of the mixing effects.Calculations of the flow through multistage machines have been madeby incorporating the new model into a streamline curvature throughflow calculation method and the improvedagreement with experimental data has been obtained.It is believed that the simpler equation system can beapplied to the flows not only in subsonic but in transonic and supersonic compressors if an appropriate modelis proposed for the apparent mixing coefficients. A basic equation system for meridional throughflow fields in multistage axial flow compressors has been deduced, containing many unknown correlation terms, which describe different kinds of spanwise mixing mechanism in aunified form. Equation system shows that spanwise mixing of meridional flows in compressors is attributedto three kinds of mechanism: molecular motion, turbulent diffusion, and circumferential non-uniformities, thelast of which includes secondary flow effects and others. wherefore the equation system unifies the two models for spanwise mixing analyzes by Adkins & Smith (1981) and Gallimore & Cumpsty ) .With three kinds of composition of introduced purely in the basic equation system, a novel, much simplerequation system, without additional unknown correlation terms included, has been. This novel equa- tion system makes throughflow computations including mixing far easier for multistage compressors.It hasbeen rigorously shown that these apparent mix ing coefficient contain all of the three kinds of mixing mechanism so so the simpler equation system can also be taken as a unified model for meridional flowswith all the kinds of mixing effects.Calculations of the flow through multistage machines have been made incorporating incorporating the new model into a streamline curvature throughflow calculation method and the improved agreement with experimental data has been obtained. It is believed that the simpler equation system can beapplied to the flows not only in subsonic but in transonic and supersonic compressors if an appropriate modelis proposed for the apparent mixing coefficients.