Abstract: Two characteristic cells are in AFA-3G fuel assembly,that is typical cell and control rod guide cell.And there are some rules on the arrangement of mixing vanes.For the two characteristic cells,mixing capability is evaluated axially from the point of the first and second kind of sub-channel with CFD method.Mass mixing and heat mixing are interaction but different with each other.Although the mass mixing in the first kind of sub-channel is stronger,the thermal capability of the two is to some tune from the point of heat transfer.In the experiment research on thermal-hydraulic performance of AFA-3G fuel assembly,the arrangements of mixing vanes should refer to the two spacer grids of characteristic cells.