Abstract: Flow separation is significant within the lateral inlet/outlet, and the flow field contains multi-scale turbulent coherent structures, exhibiting strong unsteady characteristics. This study uses the delayed detached eddy simulation (DDES) method based on the k-ω SST turbulence model to analyze the relationship between multi-scale turbulence coherent structures, flow separation, and turbulence statistical characteristics. The results indicate that: ①Under the influence of the adverse pressure gradient, the diffuser section exhibits an evident stratified flow structure, with the mainstream biased toward the lower part of the channel, while the recirculation zone extends from the channel corners to the entire upper region of the channel; a shear layer with high turbulent intensity forms between the mainstream and the recirculation zone. ②POD indicates that the coherent structures in the flow field exhibit a clear multiscale cascade feature, and the modal energy decreases progressively with increasing mode order. Among them, the large-scale structures represented by the low-order modes dominate the unsteady flow behavior. The large-scale strip-like structure with a dimensionless frequency of S_t=0.0175 is essentially a corner vortex developing along the streamwise direction, and its periodic evolution dominates the unsteady variation of the recirculation-zone boundary. ③The small-scale vortex structures represented by the high-order modes are mainly concentrated in the shear layer and are highly consistent with the spatial distribution of the regions with high turbulent intensity, indicating that the interaction and dissipative breakdown of small-scale vortices are the main mechanisms responsible for local energy loss. The findings provide theoretical support for the design of lateral inlet/outlet in pumped storage power stations.