This paper presents an active aerodynamic control method of flutter oscillation for long-span suspension bridges with a newly designed twin-winglet system. The key point of this paper is to establish the theoretical framework for active control of bridge flutter by a pair of rotatable winglets beyond the deck, of which motions are determined by the feedback control algorithm. Through utilizing aerodynamic forces generated by these winglets, this active control system can improve aerodynamic stability of long-span suspension bridges to some extent. The modeling and control of this system mainly focus on practical feasibility, in which the relative rotations rather than driving forces of both winglets are selected as the control variables in the paper. State space expression of the control system is proposed to handle problems of high order terms elimination and decoupling for control variables. Since the unobservability of aerodynamic states, model reduction technique is adopted in the design of observer and controller. The active control algorithm presented in this paper is verified by numerical simulations, and the stabilizing mechanism and energy consumption are discussed as well. It shows good effectiveness and robustness of this active aerodynamic control device in a wide range of wind speeds for flutter suppression. (C) 2015 Elsevier Ltd. All rights reserved.