Abstract: The flow field behind a rotating cylinder with Reynolds number Re = 20000 ~ 90000 and relative speed ɑ = 0 ~ 0.72 was measured experimentally, and the velocity distribution and turbulence distribution at different sections behind the rotating cylinder were analyzed. The flow around a rotating cylinder is numerically simulated by LES method, and the characteristics of the flow field around a rotating cylinder are analyzed. Finally, the theoretical model is used to analyze the flow field variation and came to the following conclusions: When the cylinder rotates counterclockwise, with the increase of the relative speed at the same Reynolds number, the position of the velocity mutation below the wake region of the rotating cylinder moves up with the increase of the relative speed, while the position of the velocity mutation above remains unchanged. With the increase of Reynolds number, the position of velocity mutation below the wake region of the rotating cylinder moves down to a small extent. Through numerical simulation, it is found that the position of the lower vortex behind the cylinder moves up obviously after the cylinder rotates, and the amplitude is large. The lower free shear layer has obvious upward movement, while the upper free shear layer has little change in position. Finally, through theoretical analysis, it is found that the upward movement of the lower vortex on the rear side of the rotating cylinder has a significant effect on the lift force of the rotating cylinder. Under the condition of high Reynolds number and low relative speed, the change of the lower vortex position on the rear side of the rotating cylinder has an important effect on the lift force of the rotating cylinder and the change of the free shear layer in the wake region.