Abstract: The flow introduced between subchannels due to directional transverse flow mixing and undirected turbulent mixing causes changes in the momentum and energy equations within the control volume, and the intensity of mixing affects the design and evaluation of thermal diffusion and the reactor core geometric structure. The rod power and grid structure respectively affect the distribution of hot and cold channels and the axial variation of flow area in the fuel assembly, which have the effect on directional transverse flows between subchannels and undirected turbulent mixing. Based on CFD method and subchannel code ATHAS-LBE, the mixing characteristics of the assembly were solved with the effect of power distributions of bare rods and grid structures. From the CFD calculating results, it is shown that a peak of transverse mixing index formed downstream of the grid structure and then the mixing index decays nearly in a negative exponential form along the axial direction. For different types of subchannel, transverse mixing is enhanced from the inner central subchannel to the outer side and cornered subchannel. The range of axial mean values increases outward from 10⁻³ to the order of 10⁻¹. In terms of turbulent mixing, except that turbulent mixing shows no significant difference under the uniform power distribution, the turbulent mixing coefficient in the corner subchannel gap is generally higher than that in the central subchannel gap over the axial range, with its average value being about 2 to 3 times that of the central subchannel gap. The transverse mixing and turbulent mixing of central and side subchannels under the half side power distribution are both higher than those under the uniform power distribution and the center-periphery distribution. From subchannel code analysis, the greater the turbulent mixing coefficients, the smaller temperature difference of subchannels can be obtained. Thus, it leads to a more uniform outlet temperature distribution of the fuel assembly. In actual engineering tests, the temperature measurement points of subchannels should be avoided near the grid structure whenever possible. The enhanced mixing formed in these areas may interfere with the measurement and evaluation of the overall mixing capability of the assembly, unless the local mixing coefficient is corrected. It is recommended to obtain the temperature distribution of subchannels at the same cross section which far from disturbances. Applying the methodology and modelling strategies introduced in this paper, the range of the mixing coefficient can be initially determined and the mixing characteristics in each subchannel are obtained, which provides as an input for the verification of subchannel coding.