This paper deals with nonlinear analyses of sheet flutter in a narrow passage caused by fluid-structure interaction. Attention is paid to the behavior of sheet when flow velocity exceeds critical flow velocity. With discretization, the sheet is modeled as a combination of mass less beam elements, springs and discrete mass particles, in which the mass of each particle and spring coefficients are calculated based on the beam model. Nonlinear fluid forces acting on the mass particles are formulated then coupled with equations of motions of structure. After that the behavior of large-amplitude vibration of cantilevered sheet is simulated using numerical computation, showing that vibration of sheet grows into a limit cycle oscillation then becomes unstable with increasing flow velocity.