The symmetry of coolant flow within rod bundle channels is disrupted by the presence of spacer wires, leading to the generation of secondary flows and vortex structures in the inter-rod gap regions, and consequently, turbulent mixing behavior is significantly altered. More uniform temperature distribution in the flow field is promoted, and the risk of hot spot formation is reduced through the enhanced turbulent mixing, which is crucial for the safe operation of nuclear reactors. Computational fluid dynamics (CFD) was employed to simulate and analyze the turbulent mixing characteristics between different subchannels in a 7-rod bundle unit with 4 rectangular helical wires. The results indicate that the turbulent mixing coefficient at the interfaces of the subchannels exhibits periodic variation along the axial direction, with a cycle length equal to 1/4 of the wire pitch. The minimum value of the coefficient is observed at the wire contact points, and it gradually increases with distance from the wires. Furthermore, the turbulent mixing coefficients at the edge-center and center-center subchannel interfaces are found to be significantly higher than those at the edge-corner subchannel interfaces. Based on the numerical simulation results, an empirical correlation for the turbulent mixing coefficient under different Reynolds numbers and pitch-to-diameter ratios is derived.