Unsteady flow and vortex-wake structure of flow around a blade are the main factors inducing hydro-mechanical vibration. A pair of plate cascades arranged transversely was taken as the research object. The three-dimensional flow field was calculated based on the large-eddy simulations (LES) when the Reynolds number was 4.8×104, and the characteristics of vortex evolution structure, lift and drag coefficients and pressure pulsation were analyzed. The flow field structure and excitation mechanism of the flow around the plate under wake interference were discussed. Results showed that the hairpin vortex structure on the surface of the single plate is staggered and the wake shows a high degree of three-dimensional characteristics under this Reynolds number. The flow structure between the upstream and downstream plates under the cascade flow regime significantly restrained the evolution of vortex structure on upstream plate surface. The force of plate was continuously switched between the regime H and regime L, and the magnitude of lift fluctuation was increased with the increase of drag. The regime switching frequency of downstream plate force under the wake interference and the amplitude of the lift fluctuation were both increased. Analysis on the results of flow and force around plate under wake interference can provide reference for active control of hydro-mechanical blades wake interaction, which would facilitate the development of the design to control vibrations and noise.