A full-order and single-parameter searching method (F-S method) for analyzing coupled flutter instability of long-span bridges is proposed based on the full-discretized model of structure. Based on the proper approximation of the circular frequency of complex modes, the characteristic equation of the full-order system is expressed as a complex generalized eigenvalue equation that contains only two variables. The equation is used for flutter analysis by solving the complex generalized eigenvalue problem with an efficient simultaneous iteration method directly. Since its computation is reliable and efficient, the application of the proposed method on the flutter problems of long-span bridges is practical. Moreover, the flutter analysis is performed for Jiangyin Yangtze river suspension bridge with 1385 m main span in the completed stage to illustrate the reliability and effectiveness of the proposed method.