The dynamic feature makes flapping-wing much different from fixed-wing. In order to study the unsteady aerodynamics of flapping-wing, a flapping-wing wind tunnel experimental system based on virtual instrument was set up, which can perform real-time measurement of aerodynamic force, flapping angle, power consumption and so on. The Strouhal number (St) was regarded as a key issue of unsteady aerodynamics. A series of wind tunnel experiments, based on the definition of St, were carried out to explore the influence of the St, which was varied by different flapping amplitude, flapping frequency and wind velocity. The lift, thrust and propulsion efficiency of the flapping-wing in different conditions were analyzed, and the influence rule of St on aerodynamic characteristics for flapping-wing was get. The experiments showed that, as St increased, the aerodynamic of flapping-wing increased significantly; and the propulsion efficiency kept in a high level when St was in a certain range.