Abstract: Active flow control effects of the circular cylinder wake flow with a Reynolds number of 10000 are investigated by wind tunnel tests. The flow control is conducted by steady suction based on a structured porous surface, and the test cases have various equivalent suction coefficient C_μ. The two-dimensional wake flow fields are measured by particle image velocimetry (PIV) technique. Based on the flow fields, the results of proper orthogonal decomposition (POD) modal characteristics, instantaneous vorticity evolution, power spectrum density of fluctuating velocity, time-averaged flow characteristics and drag coefficient estimation are analyzed and compared to evaluate the flow control effects. The results show that under flow control, POD modal energy distributions are changed, the streamwise vortex energy transportation process is strengthened and the vortex shedding mode is transformed when C_μ is large enough. The shear layers are elongated along the streamwise direction, and the upper and lower layers become parallel to each other; while the vorticity values are restrained. The dominant frequency of the fluctuating flow velocity is curbed and the turbulence fluctuation and momentum exchange effects are suppressed. The estimated drag coefficient can be reduced up to about 11% of that of the uncontrolled cylinder.