Abstract: The problems of the two-dimensional flow pasting a flexible filament and a cylinder are investigated using lattice Boltzmann method and immersed boundary method, and then the numerical results are compared with the results of previous research. To improve computational efficiency and accuracy, multi-block strategy is developed, and boundary treatment method is adopted at the boundary of cylinder. The lift and drag coefficients of filament are analyzed, and then the influence of spacing between filament and cylinder on the wake characteristics and flapping situation of filament is investigated. Furthermore, the effects of Reynolds number and flexible filament quality ratio are discussed. The drag reduction of filament is remarkable because of the downstream cylinder. Within a reasonable parameter range, the displacement of tail decreases and then increases, due to the viscous effects and the interaction of vortices. Increase of Reynolds number can enhance the movements of filament, and the quality ratio demonstrates a role of instability.