Abstract: The particle image velocimetry (PIV) experiment was used to investigate the effect of Reynolds number on the flow around a long plate with different aspect ratio. The Reynolds number is Re=500, 800 and 1000, respectively, and the ratio of plate flow length L to thickness D is L/D =3 and 6. The experimental results showed that when the aspect ratio was L/D=3, the fluid was separated from the leading edge of the plate and then re-attached to the trailing edge of the rectangular cylinder. When the aspect ratio was L/D=3 and 6, the length of the separation bubble above the plate and the reflux zone behind the plate increased with the steady build up of Reynolds number, and when the aspect ratio was 6, the influence of Reynolds number on the separation of bubbles on the plate was relatively small. Reynolds number and aspect ratio affected the turbulence characteristics of the flow field. When the aspect ratio was L/D=3, an increase in Reynolds number would lead to an increase in the pulsation intensity on both sides of the plate, while the pulsation intensity behind the plate would decrease. In addition, the turbulence fluctuation intensity of the flow field would decrease with the increase of the aspect ratio. At the same time, the small-scale vortex structure in the flow field increased with the increase of Reynolds number.