Abstract: The effects of a flexible film at the free end of a finite square cylinder on its aerodynamic forces were experimentally investigated. The width of the cylinder d was 40 mm and the aspect ratio H/d was 5. The oncoming flow velocity ranged from 4 to 20 m/s, corresponding to a Reynolds number of about 10 960 to 54 800. A flexible polyethylene film with the thickness of 0.04 mm was vertically attached at the leading edge of the cylinder free end. The length of film ranged from 0.5 to 4 cm. It is found that, the behavior of the flexible film has significant effects on the aerodynamic forces of the finite square cylinder. At a low wind speed, the film is statically deformed, which has negligible influences on the aerodynamic forces of the cylinder. The flexible film flaps at a high wind speed, resulting in a significant reduction in the aerodynamic forces on the cylinder. The maximum reduction in the mean drag, fluctuating drag and fluctuating lift reaches about 5%, 25% and 60%, respectively. The reduction in aerodynamic forces is similar for all tested films once the flapping occurs, although the critical velocity is different for the film with different lengths. Flow visualization indicates that, the flapping film induces strong vortices in the free-end shear flow, changing the alternate spanwise vortex into symmetrical, especially near the free end.