Abstract: In the process of studying the influence of a synthetic jet actuator to a backward facing step field, it was noticed that flow field perturbed at frequency near St=0.04 is quite different from that at higher frequencies. In order to analyze this phenomenon, the snapshot proper orthogonal decomposition (POD) method was used to decompose the velocity distribution data collected by PIV, and low order motions were used to rebuild the main motion of the field. The result shows that the phase diagram of lower frequency perturb is similar to the Lissajous, and describes the flapping motion of the shear layer.