The general goal of this study is to construct the closed loop control system of the reattachment process in a backward facing step flow by a row of the vortex generator jets. It is necessary to examine the dynamic characteristics of the injection jet on the reattachment process in the design of the control system. The coherence and the frequency transfer characteristics between the injection jet and the near-wall flow in the reattachment region are examined in the open-loop system. The result shows that the coherence between the jet velocity and the surface static pressure is higher than that with the surface friction velocity and the surface static pressre has better characteristics of the frequency transfer gain at the streamwise station of X/H>7. A step response of the near-wall flow is also conducted in order to find transient characteristics. It is found that the damped oscillation occurs in the surface static pressure immediately after a step change in the jet velocity.