Abstract: For the poor aerodynamic performance of airfoil at low Reynolds number, the paper targets the active oscillation for flexible skin of airfoil in order to improve its aerodynamic characteristics and flow field structures.Roe method with preconditioning technique was used to solve the unsteady compressible N-S equations and simulate the flow for NACA4415 airfoil at low Reynolds number.The aerodynamic force characteristics and laminar flow separation structures were compared by time-average and unsteady methods when the skin is static or oscillating.Preliminary studies indicate when the flexible skin actively oscillates with appropriate amplitude and frequency, the time-average lift and drag characteristics increase significantly.The separation bubble structures transform from trailing-edge laminar separation bubble to classic long laminar separation bubble.The separation position moves downstream and the separation region reduces.On this basis, the paper studied the alterations of the unsteady flow structures and pressure coefficient distributions more detailed during one period with the two states of the skin.The flow field structures and the pressure distributions keep steady in the front part of separation region at a static skin.The flow approximates steady separation and the unsteady flow phenomena like Karman vortex streets only appear at the trailing edge.Otherwise, for the oscillating skin, the unsteady vortexes generate near the separation position, subsequently move, and then shed along the airfoil surface.The flow presents unsteady separation and displays a wide range of pressure oscillation.Owing to the oscillating skin, the fluid movements are closer to the wall.The large scale laminar flow separation phenomenon is suppressed apparently.