The turbulent mixing between rod bundle sub-channels is a key part that affects the accurate calculation of thermal parameters in the reactor core,which is of great significance to improve the accuracy of reactor safety analysis.For the turbulent mixing between rod bundle sub-channels with grids,the existing research often uses the thermal diffusion coefficient to study its average effect,and lacks detailed analysis of its axial evolution characteristics.In this paper,based on the tracer analysis method,the single-phase turbulent mixing characteristics of the two sub-channels with and without the grid were studied experimentally,and the experimental results showed that the grid had a significant enhancement effect on single-phase turbulent mixing.Compared with the non-grid condition,the enhancement effect of turbulent mixing at the grid was the strongest due to the strong disturbance of the grid and cross-flow effect,the near downstream of the grid was the weakest due to the reverse cross-flow effect,the far downstream of the grid was slightly stronger than the upstream of the grid,and the enhancement effect could last for a long distance.