In this study, an experiment was performed to clarify the flow field, in which the rods were normally inserted into a main supersonic flow with a porous cavity. In the experiment, a porous cavity is attached to a bottom wall of a main duct. A thermal tuft probe was adopted to detect a vertical flow direction in the cavity to monitor the flow behavior in the cavity. The experiments show that the three rods aligned spanwise retard the downstream displacement of the starting shock wave. It also reveals that the interaction among the rods, the porous cavity, and the starting shock wave changes the flow in the cavity suggesting the strong circulation between the cavity and the main supersonic flow. Moreover, the frequency shift of the cavity flow from about 370Hz to 400Hz is confirmed during the displacement of the starting shock wave from upstream to downstream of the porous cavity.